Another wheel backspacing question

[bigblock69]

I am doing a frame off on a 68 stepside. Question is this. I am using a rearend out of a 71 LWB fleetside I bought because it was already set up the way I want it "eaton posi w/ 3.73 gears". The rearend is aproximately 2 inches wider than the original out of the 68. The front has been converted to discs. I want to get some wheels bought before installing the bed, cab, etc "just personal gratification of having a completed rolling chasis I guess". I am looking to go with 17 or 18 inch wheels, style and manufacturer unknown yet, and wanted to know if anyone has the same setup as mine and what backspacing to use. I really like the stance and tire width of project copperhead, I just don't know the width of the rearend use in that truck. Any information would be greatly appreciated.