Need some help on rims and tires. This is what I am looking at:
FRT G60-15 Goodyear polyglas 8.4" tread width
Rear L60-15 Goodyear PolyGlas 9.0" tread width

Rims Rallye 15x 8
These come in 4.50 or 4.625 backspace. Will the L60's fit ok in the front? Which combination is better for the 71 RR's, same size all around or the mix?

Not good with backspacing determination so to clear the L60-15 tires how much backspacing is needed for frt and rear to avoid clearance issues if there are any. I an keeping the stock ride height.

What car are these going on? Also any modifications that would impact wheel/tire fit (lifted, lowered, different rear axle, aftermarket brakes/hubs, flares, ect.). A pic of the vehicle now would help also.

Its a 71 Roadrunner 440-6 with 4.10 Dana 60 and 4 speed. Going with stock ride height.

My experience is with a cousin - a '70 Challenger. I'll share and hopefully others with your combo will add. In general, a 225 tire in front should be an easy fit with ~7.4" tread width. In the rear, a 255 with ~8.9" tread width should be an easy fit. Bigger is close or can hit in more extreme situations of full lock and jounce/rebound.

I think your desired tire sizes are tight for a stock setup.

Also, I think clearance in the front would be better with less backspace... around 4.25" with 8" width wheel. Rear is OK with the higher backspace.

Before the big investment in wheels/tires any way you can get a test wheel/tire? A borrowed wheel and used tire even? This would help a lot. Mount and check fit. I did this to confirm before ordering with a really helpful tire shop.