Some diameters and a summary of wheel numbers I've read, both from here and elsewhere.
235/75 R15 = 28.9" Pretty close to the original wheel diameter on a 3600
235/85 R16 = 30.5" Almost exactly the same as the optional 17" wheels you could originally order
235/85 R17 = 31.7" Picks up an inch over the original 17" option, 3% better (lower engine speed for same MPH) than the 234/85 R16 and 13% better than the 235/75 R15
3 1/2" backspace is a must (if you want to avoid using spacers) according to the prior posts on this board, although some members indicate that 4" backspace might do if you have the 1 ton tie rod ends. It's close to impossible to find the 3 1/2" backspace (Dexstar trailer wheels are the popular recommendation: 16x6 8/6.5), so it would be nice if we could squeak by with 4" as well.
Bolt pattern spacing is 6.5" which is 165.1mm, not 160mm as is sometimes stated.
Lots of guys say that Ford rims work, but I've not had much luck confirming backspace just from websites selling them. Either the data is not listed or when it is the backspace for the listed rim is greater than 4".
Grigg posted on here about using 19-19.5" wheels. He was using bias ply truck tires, but if you could put a radial on those they would be a whopping 34.7" in diameter for a 235/85 R19. That would be awesome if you could fit all that under the fenders!
