I find that focusing on backspacing will determine wheel fit far easier. The dimensions have a certain amount of co-dependence based on width, but your backpsacing will tell you far more in terms of raw fitment (but less about dish, lip, face).
As an example, I have a 1972 12 bolt rear end in my 66...which is a stepside. The 66 steps use the 50's beds still, so we may be a hair narrower (didn't the 68+ start to use a new stepside bed?).
Anyway, with a stock with 12 bolt, I wanted to run a 12" wide rim, and 325/50/15. Thats a lot of wheel on a stock width rear end, but I achieved it with custom wheels. My rims use a hefty 8.5" backspacing, which allows the 12 beadlock, and still lets me go TUCKED on the bed. I mean TUCKED, the combo is really near the theoretical limit of the step bed, the frame with the shocks and trailing arms I have now. With a slightly larger diameter wheel I could maybe squeeze a little less backspace, but it would be damn close. If I ran a lower profile tire that didn't bellow at all, I could probably sneak to a 9" BS, but I don't know if I'd want to. An excessive BS puts the load in weird places.
There is a conversion chart that helps explain the correlation.
A 10" wheel is 0 offset on approx a 5.5" backspace. So, you need to measure (or google) if a 20"x10 wheel on a 5.5 will fit.
In comparison, the my wheel mentioned above would be 0 offset @ 6.5" backspacing, which would never fit my truck.
Hope that helps and doesn't make it more convoluted.
http://www.wheelspecialists.com/wheel-specs