You're going to want about a 3 to 3 1/2 inch backspacing, offset will depend on wheel width and what final backspace you go with. Generally, a wider wheel with a given backspacing will have a negative offset compared to a narrower wheel with the same backspace specification. With the tire size you're wanting to use, a 15x7 would be perfect, with a 3 1/2" backspace and 0mm offset. If you want a wider wheel, 15x8, with 3 1/2" backspace and -25mm offset will also work. Both were OEM sizes used on solid axle GM 4x4 trucks up to '87 ('91 on Blazer and Suburban).
