So I called Fabtech and asked about wider tiers and they said no, keep it at 12.5, but I didn't ask about bigger. Can I get 37's instead of 35's to fit the 6" lift without major problems? I won't be doing major offroading, mostly playing in some mud. I need to know quick so I can change my tire order before the rims get to the place. Apreciate the input.

 

i believe everyone with the 6 inch lift is using 35's most 1500's with 37's have a body lift on top of the suspension lift

 

Not 100% sure on the '06+ kit, but the Fabtech 6" kit for the '02-'05 really measured just under 5.5" of lift and lots of people who got it knowing that 6" barely cleared 37's were disappointed as they would not go without major trimming.
I off-road quite a bit and I don't personally know anyone who has not broken something running 37's off road on ANY brand of IFS truck. I know one guy who broke down and had to get a tow on the highway, going on vacation with his girlfriend and her kid, a little under 400 miles from home. Lost 3 days of a 7 day vacation having his front end fixed. Only mild off-road use and about 3500 miles on the 37's.
So, IMO, stay with the 35's and leave the bigger tires to the solid axle guys...