Best news this week! I was kinda dreading all the cutting, straightening, squaring up, welding, etc. Now all I have to do is find someone that's already figured out the backspace thing for 15" wheels.
Not that hard to figure out. A SBP A-body 8 3/4 is 57 1/8” with the brakes, a 65-67 body 8 3/4 is 59.5”. So compared to someone with a small bolt pattern A-body 8 3/4 you need an extra 1 3/16” of backspace to put the wheel in the same place.
If you compare to someone with an A-body housing running BBP axles it will be a bit less than an extra 7/8” of backspace per side.