I am going to put in a spring relo kit to maximize wheelwells.....So does anyone know the biggest tire/rim I can put in there and the proper backspacing of rim to stuff it..:0)

Welcome Nivvy

Are you going with a bolt in spring hanger or weld in?
Are you going with a short spring hanger or a 2 inch
long one that lets you use the super stock spring that
was designed for the A-body.

I have a set of the bolt in 2inch super stock style hanger
and spring and the rearend is a 8 3/4 B-body housing on
my '74 Barracuda.
295/50 on the back with 10 inch Autodrag centerlines
Fits OK but I could have moved them in 1/2 inch more
if I tried a little harder.
The main point is there is no cutting or welding with this
setup and it's cheap...
one more thing 275/60s might look  a little cooler if you
plan on lowering the body on it

im runnin a 15x10 with 295s but my tires stick out a little bit. maybe...maybe an inch on each side. but i think it looks pretty good.

What backspacing did you use?