I used a 9" rear out of a 59 Ford sedan for my 65.  This rear has the "small" bearings, with 28-Spline axles.  This rear bolted right onto the existing Mustang springs, no issues with the spring locating pins.  Pinion yokes are a PIA (from my experience) as some use 1-1/16" cups on one side and 1-1/8" on the other side of the u-joint so you'll have to check it out.  As I switched to the T-5 transmission, I had to get a shorter driveshaft.  I also switched to rear disc brakes instead of rebuilding the 9" drum brakes.
Having used the 57-59 9" in 65-66 Mustangs in the (distant) past, the driveshaft length was fine (with 4-speed toploaders), hydraulic and emergency brakes from the 9" simply hooked up w/o issues.
I'm not sure you can still get these old rears any more.  But if you're buying a rear from one of the suppliers, I believe they can set you up for whatever you want.
Also note that unless you plan to race your car, the 8" is fine for street use.
 

Do any of these projects ever go smoothly.....you are asking for a lot Dan!! 

Good advice so fa and second the 8" for street use. I think a lot depends on the route you take.

A factory 65 Mustang 9" housing should bolt right in, finding one will be a different story. Once you do, it it will be expensive. I know a guy that paid $900 for one 5-6 years ago.

The 28 spline axle shafts and backing plates for the small bearing rear ends should be the same for both 8" and 9". 

If you buy an aftermarket housing, its doubtful to come with the tapered tubes that should be on your car now. You can spec it how you want for width dimensions and bearing sizes. Then it will be time for new U-bolts and plates. The factory width, your current bearing size, and shaft spline count should allow you to reuse you're existing axle shafts and brakes if an 8" is in your car already. If you choose a slightly narrowed housing to offset set a brake upgrade then axles come into play. I don't know that the factory shafts can be cut down. 

Specialty U-joints can get you past the mis-matched driveshaft and yoke. Finding them could be an issue and it will be more expensive than a standard U-joint. If you have to purchase a third member new, spec a yoke to match your existing drive shaft. 

If you have a MEGA horse power engine transplant in the future, but all new parts for the rear end, upgrade to 31 spline axles, get the big U-joints for the yoke, and upgrade the drive shaft to match!! 







 

This post is like somewhere someone knew we were talking about this swap. No details though of what kind of 55 Ford it came out of. Can you guys tell if it is a 9"?

 https://dallas.craigslist.org/ftw/pts/d/fort-worth-1955-ford-rear-end/7192381009.html

Raymond_B wrote:

This post is like somewhere someone knew we were talking about this swap. No details though of what kind of 55 Ford it came out of. Can you guys tell if it is a 9"?

 https://dallas.craigslist.org/ftw/pts/d/fort-worth-1955-ford-rear-end/7192381009.html

9" started in 57.......
6s6
 

Raymond_B wrote:

This post is like somewhere someone knew we were talking about this swap. No details though of what kind of 55 Ford it came out of. Can you guys tell if it is a 9"?

 https://dallas.craigslist.org/ftw/pts/d/fort-worth-1955-ford-rear-end/7192381009.html

That ad says the rear end is offset.  You don't want an offset rear end.  So 8" or 9" doesn't matter much in this case.

(By the way, I think it is an 8")
 

Raymond_B wrote:

6sally6 wrote:

Raymond_B wrote:

This post is like somewhere someone knew we were talking about this swap. No details though of what kind of 55 Ford it came out of. Can you guys tell if it is a 9"?

 https://dallas.craigslist.org/ftw/pts/d/fort-worth-1955-ford-rear-end/7192381009.html

9" started in 57.......
6s6
 

Thanks! I did not know that.

 
Wouldn’t that also be from a truck?
Axle pads are on top.

Would you run an 8” behind a 351w, stock or modified?

As for 31 spline axles?
Especially if housing isn’t stock width?
Can 31 splines be run in a small bearing housing?
I thought there is a conversion bearing for that setup too.

Jegs lists a house brand 31 spline axle for 67-70 Mustangs. I have them in my car with the small bearings. Pass on the bearing kit from Jegs. I bought it then figured out they were the wrong size to late to return. 

There might be enough splines to cut them down for a 65-66. It would take some phone calls to confirm. If you go narrower than stock, then all bet are off. Aftermarket axles could be the next stop. I can't tell you if anybody is better than anyone else. 

If you take the Daze axle build approach and use a banjo style housing you end up with what looks like a hipo 9 inch.  Its not as strong as the later style housing but it provides more clearance for exhaust.  I had one made for my old car and had a few more made that I had sold.  That was a long time ago when I use get get every 9 inch axle that came into junkyard.  Was good friends with the owner.