Hick, the kind words are much appreciated. I want to apologize for the lack of updates, you know how it goes though. Priorities and that little thing called life start to rear their ugly faces at you for some much needed attention. I'll get back to it, probably sooner than I would imagine too. Call it a brief intermission if you will...
Onto your setup. No. I did not get to drive it with the Th400. However if i've ever been called one thing it's a "research whore" and as such i've come to the conclusion over the past few years of doing nothing but reading, research and numbers crunching through various forums that the Th400, while being a superbly built transmission, is not the best choice for this application. If you take into account the GMC's permanent 15% underdrive built into the transfer your effectively looking at 6.33 axles. Not cool for engine life or mileage, and with today's build tech and newer aftermarket tranny parts there's no reason to restrict yourself to just a Th400 anymore.
As far as weight goes, remember your GMC weighs 14,000 but it also has gears to suit that weight. Not all things being equal, but the "gear to weight" ratio is basically the same as a M1008 1 ton CUCV. So "stress" wise your engine/tranny will be under the same load as it would see in a lightly loaded pickup. You can quickly see how even the so called "weak" 700 is perfectly suited for such a vehicle. Throw in a couple aftermarket parts and run a huge tranny cooler and you should have zero problems even using the GMC to tow it's rated 10,000lbs.
Tom at Alfa Heaven who I will credit for initially giving me the idea about the 6.2/700r4 swap said he had more than a couple GMC's with the setup that he used to haul upwards of 1500 to 2000 gallons of water at 70mph, routinely and with no issues. Granted, he did admit they do best stuck in 3rd whilst towing, but for normal highway/unladen cruising your bet you can use every bit of that 4th gear and lockup converter function. Actually, he told me he used to haul some loads in 4th locked up, and his trick to keeping the tranny alive was to simply stay at a speed where the fluid temp stayed constant.
Probably more info than you wanted, but I haven't been posting much lately and i've been getting olive drab withdrawals.
I think the biggest factor is your motor, like you already mentioned. But now a day's, you can build the 700 (and inexpensively I might add) to take just about anything most of us normal guys can throw at them. I believe raptor has a new one out rated at 750hp. Amazing stuff, tranny tech has come a long way. Subscribed to this thread. You, me, M1010Plowboy and a few choice others will be gathering our GMC's for a little rally someday. You can bet on it.
4L80's are WAY outa my price range! 
