Well one thing you have to remember here is that your all looking at width and forgetting about the shock mounts and how wide all the other lower control arm geometry will be off too. I guess if your car will only be tooling around the streets at no more that 40 MPH and never hit a corner at over those speeds you will be fine... lol.
All these components need to be in the same locations for Pinion angles to be correct and rear end suspension and articulation work correctly.
The 1965-1970 Pontiac rear is a very good alternative with the Fab Craft gears 3.55 installed. That rear will handle all your power up to 11 sec range...beyond that you will have to watch for the posi unit to start spitting spider gears out.
I have worked with Brad at Fabcraft on other rear end projects and his gears are top notch quality USA gears. Here is a rear that we have that is the 10 bolt cover 12 bolt ring 8.875 1965-1970. Fabcraft 3.55 gears installed.
The upper control arms are ALWAYS the hurdle in the GM rears as they aRE cast IN and require a spherical rod end and adjust control arms - we sell these to help install and swap GM rear into other year GM rears...
IE We do G-body swaps with 73-77 8.5 centers and change over all the rest of the lower and spring perches and narrow them to the same width with the Custom upper controls arms... We do this with 8.8 LTD centers and 12 bolt chevy rears too. It WORKS great.
We just never had as much interest from anyone coming at us for FULL SIZE cars. It seems to always come down to the money... One way or another you will have to spend some money to get a stronger rear with more selection of parts.
We are working on a GM 14 Bolt 9.5 3/4 ton truck housing to solve the super power needs of the 800+HP cars. It is Slow going as we have to fit it in between all the regular BILL PAYING orders and parts.
Jim JD