So I bought an 8.75" B body housing, axles and perches for $200...

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.
The good news is you shouldn't need to narrow it, but I would still make sure it's straight. Combine an unknown history with the 8.75's knack for getting out of square, and add in the fact that it's sixty years old makes it cheap insurance to just have it squared up.