6 X 2,000 lb is 12,000 lb. The 2 ton truck should have a safe GVW of 16,000lb (check tag in door) that leaves only 4,000 lb for the truck. I think the empty truck with a simple flat bed will weigh 5-8,000 depending on length and other details. How much do you really need to haul?
If your truck is really nice as is might just leave it as is. Or if making modifications just do bolt in stuff, no significant changes to the original that can't easily be undone.
The 6Bt will be far from bolt in, it is quite a bit longer than the GMC engine, check with your tape measure. A 4BT should fit without much if any sheet metal modifications. So will a 302 GMC, the simplest engine upgrade.
If you plan a different rear do you want it to match the front wheel bolt pattern? If so the start that decision process with what you'll do for the front axle/brakes. there is a fair amount of reading here already about such changes and swaps.
Grigg
