Stock hight or lowered? You're going to be very limited on what will fit an older truck in that rim size, most of the 20s on the market are made for late model trucks with rather deep backspacing. You'll also need to have the brakes upgraded (disks, preferrably 4 wheel power) due to the etra weight. There will be ride isuues, as the shorter sidewall of a 20" tire will not absorb as much of the road shock as a taller profile tire. Don't get me wrong, I'm not trying to discourage you, just letting you know what to expect.
I would suggest that you take a look at a few of the wheel manufacturers sites to see what they offer. From what I can find for tires, something that will fill the fenders, with stock suspension, would be a P255/50/20, which is 30.1" tall x 10.6" wide. I will give you one more thing, the tire/rim combo you're thinking is going to be expensive, don't be surprized if the total doesnt exeed $2000 or more.
