Does anyone know what the bolt pattern is for the 84 caprice? I think it 5x4.5
Also does anyone know if a 17x11 or 17x9.5 wheel fit in the back wheel wells
any help would be fantastic

It is 5x4.75 - I don't believe a 9" or wider wheel will work. P255/60R15 tires on a 7" get close to the frame, but clear it.

I've seen guys with 15x10's. what backspacing do you think they would have?
Would a 17x8 work? What backspacing should I use? I'm trying to build a budget street machine that I can take autocrossing once a year.