To correctly determine backspacing, lay the rim with the surface that butts against the hub/brake drum up. (On a whitewall tire the whitewall would be on the ground). Lay a straight edge across the rim (not the tire)...outer rim edge to outer rim edge. Measure the space vertically from the mounting surface to the bottom of the straight edge. That's your BS.
Determining your exact tire fit is a bit more complicated. I found that bolting a 14" piece of 1/8 flat stock to the mounting surface (rear wheel removed) and then clamping a piece perpendicular (like a "T") can be used to measure the space needed on the inside (bed wall side) and to the outside(fender). The tire sites give you the dimension of the cross-section of various tire sizes. Let's say you were using a 7" wide rim....and your differential is 62" from mount surface to mount surface. A rim with 3-1/2" BS would center the rim in the AD wheel well. If you divided the cross section in half you could make your perpendicular arm the same measurement and rotate the drum to make sure the mounted tire would clear.
I've found that with an AD 62" mount to mount diff...that an 8" rim with 4" BS, up to and including a 255/70/15 tire will fit. This is assuming your frame is straight, the diff is centered, and your box is properly centered. On the front a 7" rim with 4" backspacing and a 235/70/15 will give you a full wheel-well, between 3/4-1" tire clearance from both the stock and upgraded tie rod ends and clears the 5 lug disc brake conversion. This is the combo I run on my truck.
If I had it to do over again, I'd go with 7" rims with 3-1/2" BS and 235/70/15's all the way around. Hope this helps.
Dave
