Wheel Back Spacing Question

[Boomscatty]

Hey! Wondering if anyone out there can shed some light on what backspace/offset I can run on a 20x9 inch wheel in the rear of my 1960 C10 that is static dropped 5 in the rear with a 72 chev axle. I would measure but the truck is at a body shop so I thought I would see what people might know or recommend out here.

Also the front is still the Torsion bar set up but with a disc brake conversion, any recommendations as to backspacing for 20x9's up there as well???

Any help would be greatly appreciated!!!