Need rear wheel backspacing suggestions for 65 valiant with mini tubs and inboard spring kit

What rear? From what car? What bolt pattern. Is the mini tub finished?

I'd get ANY wheel laying around the house with the right bolt pattern, measure the backspace, bolt it on and see what changes need to be made for a bigger wheel.
See how close it is to the springs, does the new wheel need to be closer?
We would have NO IDEA what would fit, without LOTS more information.

(I've got a pair of 10" wheels [you do know that a ten inch wheel is really at least eleven inches wide, right?] that have 4.5" bs, and a pair of 10.5s with 7 1/4" backspace. Can't tell which would fit.)

Mopowers covered it while I was typing....slowly