Good thread as I learned there is something other than 19.5 wheels at the 4,500lb rating. Highest I have seen is 3,800lbs per wheel on a 17" rim - American Racing. The 18" rim @ 4,000lb rated tires are pretty big at 34.5" tall...
Faststreak, I would go with the more aggressive tread like a toyo M608z, but that's my preference. You do need to be careful of tire ratings and size, and it seems like you are doing your homework.
Are you sure you can clear a 245 on your GMC. My buddy needed a lift in the front to clear 245 19.5's on his 2012 GMC 2500HD...