'57-'59 Ford/Lincoln/Mercury passenger cars will have the correct width 9-inch rear end to fit your '65/'66 Mustang. These rear ends will have the small bearings --same axle bearing size as all Mustangs '65-'73. You can swap your '65/'66 Mustang 8-inch axles drum brake assemblies onto these 9-inch housings.
The '57-'59 Ford station wagon and Rancheros will have the same width 9-inch housings as the regular F/L/M passenger cars, but will have large axle bearings instead. These rear ends also have the large bearing pattern drum backing plates and these are not interchangeable with the small bearing rear end housings.
This is important because finding new drum brake replacement parts for the old '57-'59 Ford 9-inch 11" x 2" brakes is very difficult. With the small bearing 9-inch housings, you can use your 8-inch rear end brake parts. With the large bearing 9-inch rear end, you're basically out of luck.
Another major problem is even finding a '57-'59 Ford big car 9-inch rear, or a '75-'80 Granada 9-inch drum brake or 9-inch disc brake rear end, or a '77-'80 Lincoln Versailles 9-inch disc brake rear end. If you do find one, it's likely to be
very expensive and also likely that many of the parts will need replacing. --Not that none of these rears exist in any of the salvage yards anymore, but many salvage yards don't operate like they did just 10 years ago. Anymore, most are just simply scrap metal yards and useable/rebuildable cores quickly end up in the crusher and forever lost.
There are 8-inch rears that take a lot of abuse and then there are those that get broken with essentially stock engines. It's just a coin-toss as to how long one will last. A beefed up 8-inch is strong but a 9-inch will always be stronger and has the parts availablity to be several times stronger than the that of the strongest 8-inch.
You can search the salvage yards first to see if you might be lucky enough to locate the 9-inch that's right for your Mustang, but my advise would be to buy a custom width housing and some good custom 31-spline axles then hit the salvage yard to get a 31-spline 3rd member from a Ford half ton pickup. The pickups ['73-'85] commonly came with 3.00, 3.25, 3.50 and 3.70 rear end ratios. --you might even get lucky and find a 4-pinion 31-spline Traction-lok differential in one.
After that, outfit it with some modern disc brakes and you will be in business.
9-inch rear end info links from my website:
The Ford 9-inch Rear End
Differential Types
Third Member Cases
Third Member I.D. Codes
RPM-charts
Torque Multiplication
