Backspacing for 15x8 6 Lug Wheel on ‘72 K10

[Pekoe1111]

I have a 1972 K10 with 15x10 Wheel Vintiques Series 12 Smoothies. The offset is pushing my BFG K02 33x12.5x15 tires out further than I like.

I am looking at 15x8 of the same wheel but they have a 4” backspace. I need these wheels to clear my front disc calipers.

What’s the right backspacing to bring my tires in but still clear the calipers?

I know the ‘73-‘87 Rallys are the right fit.

Thanks
BB