A heads up for you though, a leveling kit will stiffen up the truck some, which may or may not be what you want.
Thanks again for the feedback. I would expect a little difference but I know, if done right, a truck can be leveled and still have a pretty good ride quality. If I'm not mistaken, the GM kit is basically the components from a Trail Boss or even a ZR2 that aren't yet installed on the other models, correct? And those trucks still ride pretty darn good. I've had trucks leveled just by tightening the torsion bars (much stiffer ride), trucks leveled with spacers (cheap but decent ride), and the last brand new truck I bought (not a GM), I had the dealer install the "factory" leveling kit and larger tires before I even took delivery and still enjoyed a supreme ride that was hardly discernable from the test drive I did before purchasing the truck.
Check the overall diameter of the tire, that is the most important aspect. Most 35" tires are actually in the high 34.XX" range and will fit fine with an 11.5" width. Some will fit with a 12.5" width, but tire OD really comes into play at that point.
The manufacturer spec on the OEM Goodyear 275/60R20's is a diameter of 32.99" (let's call it 33"). I'm pretty sure a 35" diameter tire would fit as long as it's not a 12.50. But I'm thinking more along the lines of a LT tire that's maybe not quite as tall as 35 but would be slightly wider than 11.5. I'm seeing some specs on some of the Nitto offerings that are spec'd at close to 34" tall and 11.75" wide. I'd rather have a tire that is a little shorter but a little wider than to just to as tall as possible and sacrifice width as a result.