You might shoot for a 9" rear. which is what your car should have come with. They were widely used in so many cars by Ford, and hopefully you can scrounge one up. Check everywhere, I see them on Facebook Marketplace a lot. It's a shame that I'm not ready yet, Winter being here, to install my new 351c in my Mach, as I will be pulling out my stock 351c, and will be offering it to someone like you who needs it. That won't happen until next summer. So far as using a 351M / 400, if a high winding performance engine is your target, the 351C is the engine of choice. The 351c has much more performance potential. The 351c Clevelands at high rpm do need to have lifter oiling issues addressed for rpm useage, but the 351m and 400 are even more critical, in that their main bearing journals are larger. Good for bearing torque loads, but, high in bearing speed and requiring more oiling attention at rpm than a "C" does, and of course, longer rods, again torquier, but not rpm friendly. ( Interesting, that 427 FE and 351C engines use the same diameter main journal size, Ford had rpm in mind). Wiring? That can be so frustrating, as , there was a time when these cars were just old cars, not worth more than $200, and people hacked everything, especially wiring. I'm a bit anal about wiring, I want it to have the correct connectors, be the right guage and color. Just the desire to add an aftermarket stereo system usually ends up butchering the wiring and door panels/ package tray, and fixing that today can run into unplanned expenses. I feel that I was fairly lucky in that I bought mine used when they weren't selling for much, and the previous owner took care of it mechanically, as well as stored it garaged. As for your project, I hope we can all help you get squared away here on the boards. These cars are fun to drive, and although Ford sold more of these Mustangs than Chevy sold of Camaros in '71-'73, you don't see the later Mustangs as much as Camaros and the earlier Mustangs. There's pride in fixing up and owning a car that not everyone has.
