I have a built 700R4 behind about 400hp and 425lbs of torque in my 81 K10 (a built 350 engine). The tranny has handled it just fine. I can pull my dad's big dual horse trailer in OD (with two horses in it) on the level for about 13.5 mpg. Going up hills I grab third gear when my tranny temp guage starts climbing a little. I think the trailer goes about 8000 lbs loaded.
My suggestions (and I have worked with 700R4's for while - they are better than most people make them out to be) are to buy a new towing 700R4. You also HAVE to ditch the full time 4WD if you want any mpg. Just pick out your transfer case first, so that you can order the tranny with the correct tailshaft installed.
You should be able to do both for, at most, $2500. That's with a new tranny and a used transfer case.
Chris