hello ,  I am not an expert on this , but i had the same issue with overheating caused from ac add on.
until i replaced the old rad . with new 3 rows like yours .  i got only engine  machined , no power add. 
What i can say is  the thermostat could not working properly  . Before i was using 195 and temp was like yours going up after pull .     i found regular that your temp is rising when you are metal the pedal .

But there are many factor that could go wrong :  radiator dirty , thermostat not opening completely, water pump , some debrits in the circuits that close some little passage or ( hope not ) wrong gasket ( head and intake wrongly mounted ) 

just a question : you trust the water gauge or you are using a serious temp sender and reader ?
Hope it helps
Bye 

I did replace the stat . I am now using the Mr.Gasket 180° one supposed to allow better flow when open . I did verify temp with a heat gun read 210 on upper radiator hose . Not sure if spark plugs would help but I am running autolite double platinum and was gonna switch to motorcraft copper based plugs . I read somewhere our 302 don't like platinum plugs .

Last edited by onenicefastback (8/10/2018 5:25 AM)

Kind of off topic, but nothing actually likes platinum plugs.  Platinum plugs are used so car companies can claim "goes 100,000 miles before needing a tune up".  Regular copper plugs will run better, they just need to be changed every so often.  Maybe every 20-30k miles.  Honestly, with the kind of miles most of our cars see that is in effect almost forever. 

I don't see the plugs being the source of your hot running. 

Are the headers ceramic coated?  If so that's why your temps are reading so low.  The collector's hot because you've got all four cylinders converging, instead of one pipe that only fires every other revolution. 

What fan are you running?

One more thought....Maybe your lower rad hose is collapsing.  at higher speeds the water pump will pull more and that could suck the lower hose to a  partial collapse.

You do know the 5.0 heads have water jacket ports in different location than the early 289 and 302 blocks, right?  You have to drill some new holes to make water transfer between the two to get best flow.

Hot enough for me to keep an eye and coast as much as possible. Everything is stock. Could be bad radiator, could be engine running too hot? I just dont know

i have a similar issue. Running hot on highway but not in idle or traffic. I am starting at timimnng

my lower radiator hose already has the spring in it so its not collapsing I know. I am making sure my timing is right at 2500-3000 RPM .. its suppsoed to be at 36 total usually..

From reading various posts on different forums as well as the series in Hot Rod magazine where they fix chronic problems in readers cars. Two things seem to pop up. Most stock replacement water pumps have poorly designed impellers that just don't work very well. I also hear complaints about 3 row radiators over 2 row radiators. I hear 3 rows are more restrictive to air flow and some times just don't cool well. As the others said make sure your lower hose isn't collapsing. What are you using for fans? I think sometimes with electric fans and their shrouding they hinder air flow. On my 66 I'm using some cheap azz ebay 2 row no name aluminum radiator with a PRW Eldlebrock knock off aluminum WP. I should have read the fine print better. All the pictures and specs were exactly the same. I thought it was a B package pump that was missing the original box and coulnd't be sold as a new pump. Boy was I pissed when it came, It was clearly my fault but anyway I'm stuck with it and it works great and it's been 100% trouble free. As for a fan I'm using a CL special I paid $65, It's a late 60's early 70's GM 18" thermostatically controlled clutch fan with a 75 Impala clutch. Despite not having a shroud which it should, it runs cool as a cucumber, 195* Tstat.

I'm going to tell you something about those hand held infra red guns. They will only tell you something is hot. I'm a electrician by trade and need to take CEU classes. One class I took on infra red there are so many factors that will skew readings. First that little red dot means squat. The farther you get away the bigger the field it reads and it picks up what ever is hottest in that field, not what te little red dot is on. Different surface textures and colors effect reading. You can have two completely different things at the same temperature and get two completely different readings. the guy giving the course said he carries tape around with him and uses that as a constant with a FLIR.

Last edited by Huskinhano (8/15/2018 2:21 PM)

how is the water pump?

American muscle fan , water pump is new about month old .

I did change gasket . Still ran hot . Hoping solved now but weather is killing me .

Do you have a spring in the lower radiator hose? What you describe is the same problem a friend had with his Ford. You see, when the engine slows down, the radiator pressure 'puffs' the hose back to normal size and flow isn't restricted. Try to squeeze the lower hose when it's cooled down-if it squeezes much at all, it's that spring, which is made of thin steel wire.