Some things to check out - on no particular order.
Lower radiator hose collapsing inside.
Stuck thermostat.
Clogged radiator- check it again..
Long shot, but slipping fan belt.
Check these things and everything else before cutting for a new radiator.  Your setup should be good to over 100 deg easily.

A couple of things to think about if its getting hot at highway speed.  If its cooling properly around town this isn't a typical overheating issue, so there being a problem with the t-stat, or radiator is unlikely.  Not impossible, just mot where I'd start looking.

1.) How fast were you going and at what RPM?
2.) What trans do you have?  I'm assuming its a C4, which means no overdrive.  Running fast in an older car trying to keep up with modern traffic you tend to be turning a lot of RPM, which creates a lot of heat in the trans fluid.  If you don't have an auxiliary trans cooler you need one.  Possibly what's happening is that the trans fluid is so hot its causing the coolant temp to go way up as the tiny tank inside the radiator tries to cool down the trans fluid.
3.) I don't know much about the Flexalite fan, but 2800cfm should be enough, IF its actually rated at a real world static, meaning that's the flow through a radiator.  Most fan companies don't do that.  The cfm rating you get is based on the fan in free air, which is an absurd and meaningless way to rate it.  They do this so the number sounds big.  To my knowledge SPAL is the only company that actually rates their fans properly.  Here's a quick test: what size fuse is in the system?  If its below 30 amps there's no way that fan moves 2800 cfm.  It takes 10 amps +/- per 1000 cfm moved through a radiator.  If it has a 30 amp fuse, try putting a 20 in it and see if its pops.  If it does then you probably are moving the started cfm, if it doesn't then they sold you a line of BS.
4.) I will say the a three row copper/brass radiator is a good idea at a minimum.  Aluminum is going to cool better though.

Question for everyone in regards to some of the answers I reviewed
In the 60s speed limits were higher than a lot are today. The mustang was made and sold very well with its gearing and cooling all across the USA and other places. This one was made and sold in Fresno CA,  So  if the cars transmission will cause it to overheat at freeway speeds why did they keep building it with steel fans and small radiators? Or is there something I missed?
Also there is know way that the blades are flating out on this fan as they are very stiff and have a ring on the outer edge. We used to buy stainless steel fans just because the blades in fact did go flat at hi RPMs. I trust all of your input. I believe that if anything TKO mite have something with the trans temp as we had been driving at slower speeds for about 2 hours before we got on the freeway. I will back flush the radiator and think about adding a trans cooler to the old car as it will not hurt. Also the trans filter has been changed and really was not dirty and there was if any almost no resido or deposits in the botom of the pan and the fluid was still pinkish in color with no burnt smells. This car was serviced for its entire 120K miles at the delairship in CA where it was bought in 65.  Thank you all.

Last edited by Cab4word67 (7/17/2019 8:40 PM)

Just ordered a spring. hose is soft
 

Sounds like the fan should be up to the task. 

A clutch fan is thermostatic.  There's a spring on the end of it that is affected by the heat coming off the radiator (why the distance from the clutch to the radiator is so important).  So, the answer to does it slow down at higher speeds is: maybe.  Depends on how hot the radiator is.  In theory with more air moving through the radiator the need for a fan should be considerably less than putzing around town. 

AC is going to add to the heat load.  Its' also going to add another layer of core for the air to get through, because the condenser is mounted in front of the radiator.  I've found that typically AC is going to bump temps by about 20-25 degrees. 

I have to say though I'm really starting to lean on the trans as the culprit.  The fluid doesn't have to overheat to cause the coolant temp to go up.  Trans fluid has to get north of 275 degrees to change color and start smelling burnt.  Junk in the pan is a sign of either lack of service or clutch wear caused by loading (towing, etc.).  So the fluid condition to me isn't surprising, but doesn't rule out the trans as the culprit.  The other side of it is, a trans cooler certainly isn't going to hurt anything.  In an non OD auto vehicle driving at highway speeds its cheap insurance. 
 

Bolted to Floor wrote:

Cab4word67 wrote:

In Oregon ……..  When out on the hwy the other day (85 out)  temp went up to 215+ and stayed there for about 5 miles till I got off the freeway and temp started to come back down. Should I be concerned.

Out of curiosity, was this flat highway or where there hills/mountains involved? I ask since I was in Utah recently with a rented minivan. When I started up the 5-6% grades, the temperature would climb on the gauge then go back to the normal range once we leveled off. 
 

That's the perfect storm because you are throttling the engine for power to maintain speed up the hill, which is creating heat, and then the trans is working harder too, adding even more. 

Aluminum is always going to cool better than copper/brass.  Aluminum radiators usually use larger tubes, and so they have two rows instead of 3-4 for copper/brass, but the cooling is still better.  A new vehicle hasn't used a copper/brass radiator in over 20 years at this point.  An aluminum radiator is lighter and can be made smaller and still do the same job as a heavier and costlier copper/brass one. 

Today I talked to Geoff at Champion Radiator and told him the story. he thinks that at freeway speeds I am damning the air inside the shroud .  Because the fan is only going say 35 mph and the car is going 60 mph.  Thats why I cool at low speeds. Recommend installing vent flaps in the shroud. So I bought a set of flaps and a new 2 core with 1" tubes just because. trapping the air makes good since.

Air damming can absolutely occur.  High end fan/shrouds combos typically have vent flaps in them.  The air just packs in there and the fan just keeps chopping it up, which continues to heat it as well.  Most modern cars don't have this issue because of how little airflow they actually get through the radiator.  Look at how small the grill opening is on most modern cars.  Instead they are using aerodynamics to move the air over or around the car to improve mileage.  This is why those fan/shroud setups don't need flaps.  On our old cars with huge grills they might be necessary under certain circumstances. 

Still waiting on the spring. So I moved the shroud out around the top and sides almost 1 " took it out and found that it still is not cooling. I went back tot he drawing board. I did not have this problem until about a week ago. Pulled out the temp gun and the rad is 185 + and the motor is hot like the temp gauge 200-210. As soon as it cools down we are going to look at a new T stat.

Not sure, but one thing I will say is does it have a bleed hole in it?  If not drill a 1/8" hole in the flat metal surrounding the cone in an area where it won't be covered when its installed and orient it to the top.  This allows any air in the system to bleed past the t-stat.  Air pockets in the cooling system can cause issues, and this solves that potential problem. 

They worked fine back in the day, yet overheating issues seem to be one of THE most common problems people have on this and other forums for older cars.  Sure, there's a lot of reasons for that, but I think mostly its that we demand a lot more out of these vehicles now than when they were new.  50 years ago traffic was rare.  There were far fewer people and thus far fewer cars.  I can think of the changes in my area just in the past 20 years.  The number of lights on just one stretch of road I commonly drive went from 2 to 6.  Traffic has gotten out of hand.  Trips that used to take 10 minutes now take 20, etc.  The back roads have been ruined as shortcuts because everyone seems to think they should take them.  Summation, cars sit a lot more now than they did 50 years ago, making cooling harder in stop and go traffic, especially in the summer if you live in a state where it gets hot.

Then the highway speeds have done nothing but go up.  Our speed limits on I95 are 55-65MPH, but people do 75-85, so unless you want to be the slowest guy on the road, and drive exclusively in the right lane; that's the speed you better be going.  20 years ago the average speed was more like 70, and I'm betting 50 years ago it was a lot closer to the speed limit of 55.  Modern cars have overdrive and are more aerodynamic.  Its a lot easier for them to go faster, and it asks a lot of an older car to try and keep up. 

In my opinion and experience if you want to drive an older car like a modern one you may have to update the equipment. 

I go back to the fact that every modern car on the road today has an electric fan.  I've yet to have one fail, and I've yet to have a modern car with one overheat.  It just flat baffles my mind that some people think nothing of installing EFI, rack and pinion steering, and overdrive transmissions, but for some reason an electric fan is too modern and doesn't belong on an older car.  Carburetors, manual steering, and 3 speeds all worked fine back in the day too.  If someone had a problem with their EFI would there be calls to throw it away and install a carb? 

We can debate the merits of mechanical vs. electric to death, Lord knows I love to argue (everyone else does too at this point), but its not really helpful to the guy who's car is overheating.  If the question were: "what kind of fan should I run?"  Hey, its open season.  BUT that wasn't the question, and I'm with Hornman on this one, before we break out the parts shotgun let's try some simple stuff first.  The OP determined that his t-stat is working, still haven't heard if he got that spring in the lower hose yet, or installed his trans cooler yet.