rear wheel size and backspace requested

[1971short4X4]

hi, i have a 1972 chevy half ton 2 wheel drive fleetside (coil springs) that is lowered 3.5 in front and 5" in the rear.
(i say 1972 b/c i belive this rear end is wider?? then the earlier 1967-1969 ones)

I am struggling with the rear wheel size. (up front i am am happy with my 18x8 wheels and 255/45/18 tires. In the rear i would like to run a 275 or 285 tire on an 18 inch wheel.
a) what width rear wheel can i run with this 5" drop on a fleetside (9" or 10")
b) what backspace do i want with 1) a 9" wheel 2) a 10" wheel

i want the rear wheel to not rub outside or inside and i want it out as far as possible (don't like the tucked in look

thanks so much for the advice