These trucks like zero offset rims. On an 8" front that would be 4.5" backspace. On a 9" rear that would be 5". The 11" rim would be 6" backspace, but you will have to work a ruler and make sure that will work with whatever tire you have in mind.
235 55 17 on front of my truck and 255 55 17 on rear. Zero offset 8" front and 9" rear. Stepside on my truck limits you quite a bit on rear wheel width as well as being dropped 7" with leaf spring flip. Front is 4.5" with spindles and springs.
Wheels are a very personal thing, but are you married to the 20's? I think it's just too much. I think 18's are probably the sweet spot. 235 50 18 and 285 50 18 on 8 and 9" wheels respectively would be sweet.
