Lykos
Super Trucker
285/70/17 (32.7" nominal diameter) will probably fit stock. Nice thing about the GMT-800 is that it is super easy/cheap to lift. You can put a very good lift on it for under $400 and do all the work in your driveway** and most of that cost, about $320 of it, is high quality shocks. I used Bilstein 5100's at ~ $80/ea. The rest of the lift parts are stupid cheap. Keys should be around $35, spacers maybe $40 - $50. If you want to opt for new springs in the back that's another $60 or so. I'd go for extended sway bar links, too, that adds another $10 to each corner.
.
They're great rigs, all things considered. The space can't be beat (see my tag line!) Sounds like you've rebuilt the one major component that is known to be weak (the 4L60 transmission) so you should be good to go for a long time. Welcome to the Mafia!
.
** I say 'do it in your driveway' - unless your keys are rusted in place like mine were. I ended up taking mine to a shop where they charged me about $150 to use a power hammer to knock the old keys loose and install the new ones.
I've taken your post to heart. This is exactly the route I'm taking. Thank you!