I was hoping to use the stock 18" wheels if possible. I like the look plus more $ for 5 good tires. What sort of specs should I be looking for if I decide to get new wheels? I love the classic D slot steelies, that would be really cool if I can find some 17s or maybe even 16s in the right spec. Plus they're pretty cheap. Found some cheap 16s with 0 offset and 4.25" backspacing, think that would rub too bad with some 33s with some plastic trimming? I can live with a slightly smaller tire if I'm going to have a lot of poke to run 33s.
I don't think you can use 16" wheels on a 900 because of their rotor size. Most factory wheels have a 31mm offset and some of the larger GM Accessory wheels have 24 and 27 millimeter offsets.
I spent the last couple of weeks mulling over the same thing. Strongly considered going with the early Sierra All-Terrain wheel (5366 or RPO RGA) which are 18 x 8 with a 31mm offset.
Then it was on to tire size! I was thinking 265-70-18 and 275-70-18. The former fits no problem with a level because plenty of guys on here have run 285-70-17s which are a tenth of an inch taller. The latter tire is 33.18 inches tall mathematically and would probably require some trimming even if leveled but I'm guessing. My first choice would be to run the 34 x 10.50 x 17 BFG KO2 which are 33.5 inches in diameter but don't know if they would fit either on a leveled truck with no trimming. They are quite expensive.
In addition to cost, they add quite a bit of weight to the truck and take away from its payload. In a KO2, they are 56, 57 and 64 pounds respectively.
In the end, I decided to just run another set of stock size KO2s which cost about the same as Defenders and are just under 46 pounds in a Load Range C tire.