back rims and tires fit for 72 GP
 

I guess a 15x8 with 4.5 BS and a 275/60 is probably the safest route for a big tire/rim for the back?
Thanks.

Mine are 15 x 8 295/50 on the rears. Not sure what my backspacing is though.

That is what my '72 has on it. You can see there is plenty of clearance at the lip, even with the shorter springs I'm running.

275/60-15 will fit with ease. As someone else mentioned, the 295/50-15's will also fit. Tire selection and tire performance in those sizes completely sucks though. After running 275/60-15's for a decade and a half, I presently have 275/40-18 on the back of my '70 GP. I now no longer have the staggered front and rear, because I run the similar diameter 245/45-18 and 275/40-18. My car drives and rides like a dream now.