20” wheels to 15” wheels

[epfatboy]

Yep, 20” to 15” is the direction I’d like to go.
Want to do this on a 1974 C10 short bed step side. Truck has a 4” drop.
Current sneakers are 275/40R20 front and 315/30R20 rear, no rubbing. Mounted on 5x5 Riddler rims..

WITH UNKNOWN Backspace.

Anyone have some experience with what fits my combo in terms of a 15 (ish) rim, I assume a 15/10 5x5, really looking for BS (or offset) and what rubber your running on a step side.

I’m buying new so I have some option, like I might not hate the idea of a 16 or 17” rim but I do like a meaty sidewall.

I can measure, and will, just being lazy and want to window shop.