Originally Posted by Welby74Nova
I've got a TH350 Tranny and have no idea about anything to do with the rear-end. I was thinking about going with a 700R4
A TH700R4 or TH2004R can be made to handle 500HP. But it'll cost you accordingly.
A TH400 would be the least expensive way to go, but no OD. There are Gear Vendor OD add-on units, also expensive. I'd pass on a TH350. It can be made to handle 500 HP, but would cost more than doing a TH400.
You didn't mention torque converters, but that will be a vital part of the equation when you get that far.
Besides the 8.5" diff you now have, there are several manufacturers that have 12-bolt diffs (OEM 12-bolt units are going to be awfully high, and hard to find for no superiority in strength), Ford 9" and even Dana 60-type diffs/housings available. Again, for a price.
The advice to use the 8.5 "corporate" rear end is sound. The weak points can be corrected and the diff will then be good to go @ 500HP.
Hooking that 500HP to the pavement is another thing- Nova's aren't known for having large wheelwells out back, so plan on rolling the inner lip back and possibly doing more 'surgery' as needed.
A lot of guys will just bite the bullet and install mini tubs and relocate the shocks and/or springs to provide the room for some serious meat.
There are a lot of aftermarket parts for helping the Nova hook- you'll get more acquainted w/them as you move forward.
Depending on how much time and effort you want to put into the back half for traction, you could get by w/less- but careful measurement of the rear wheel offset/backspacing will be necessary in order to fit as much tire as you safely can.
195/75-14 was standard, I believe. About the biggest rear tire that will fit w/o cutting is 275/60-15 on 15x8 wheels with 4.5" of backspacing. These were Rally wheels, and will barely rub under some conditions.
But all sorts of things can alter this, including worn or sagging springs mostly- but also body mounts- actually ANY worn rubber suspension components for that matter, collision damage/repair, modifications, etc.