Any 65-70 Mustang 9" will fit. 65-66 Mustsng rear axles are about an inch or two (can't remember the exact amount) narrower than the 67-70 rear ends. Truthfully, factory 9" rear ends are not nearly as common as they used to be. I would suggest an aftermarket housing. I converted the 9" rearend out of a late 1970's F150 to fit my Mustang. You do not want a wider rearend. You will make it difficult to fit wheels and tires. A 3.50 gear with your transmission would be a good choice. 28 spline axles are more than sufficient for your power level. Nothing wrong if you want 31 spline axles. I would suggest a TruTrac instead of a clutch style traction device. 8.8 rearend is another option for you.

 

Good advice and info right there^

Automatic transmission... unless you‘re running slicks on a prepared racing surface, even with stroker torque, you’ll never break the 8” that‘s probably in it now. The pinion bolts are the weakest link, and that‘s a cheap ‘n easy fix (studs). On street radials, 28 spline axles will put up with more than you think. And, 8” is significantly lighter and costs less parasitic drag compared to 9”

All that said, if you do go to 9”, don’t screw around. 31 splines, big pinion support, TruTrac, blah blah blah

 

A 9" rear end alone will suck away 15% of your HP(its a power hog, the 8" is marginally better) 428*0.15=64.2HP loss through the rear end alone without even bringing the transmission into it, so even if you dont count the trans the most you will have at the wheels is 364HP....but HP doesn't even matter when it comes to rear ends...what matters is torque(of which you will have less than HP), tire stickiness and general abuse. I doubt I would even bother to upgrade to a 9" in your shoes to be honest. The only reason I put one in mine is that I needed a rear end that was the correct width and it was easier for me to find a 9" near me in that width than a factory 8".

There are a lot of things to consider in building a 9" rear end....as I already mentioned, its a power hog...31 spline axles and torsen-type LSDs make that even worse. I will never make enough power to need 31 spline axle so there is zero reason for me to ever add the extra rotating weight they would add. I have to change the gears on mine to a reasonable 3.25 anyway...so to reduce friction power loss I will be running REM polished gears and likely lightened as well to reduce rotating weight in an effort to reduce that 15% down to something like 11-12%. An 8.8 built using 9" housing ends would be the better option for efficiency(the pinion intersects the ring gear directly on the 8.8" rather than offset like the 9" which results in less power loss)...but an 8.8" has a cast-iron center, which means should you ever go 3 or 4 link, welding brackets on becomes problematic unless you go with a full truss setup.

In short...I wouldn't touch the rear end until you get the rest of the car the way you want it, that way you can actually make an educated decision and avoid wasting money if the plan changes.

 

Thank you all. Based upon the information here I ended up searching for and finding a reasonably priced 8.8" 3L73 (from 2001 Explorer), Code: S613E, and 59.625" wide. Thank you!