My 87 C10 2wd came with 15x8 Rallys and is stock height

Anybody know what the backspacing on those rims is? For 15x8 they seem to sit very far in.. i would say it's greater than zero.

If I go with a 15x10 rim with a negative and stay under or around 30 inches high or lower, will I be able to run a very wide tire all around.. say a 12 or 12.5 inch wide.

I'm thinking 295/60/15 or 315/60/15 .. even in the front.