It does sound like a blend door issue. Is the return hose from the core hot all the way to the radiator? Try a lower temp thermostat or drill a couple bypass holes in your current thermostat. The lower temp stat or holes would promote more flow through the cooling system. I have low temp stats...
I have a New Carbon Fiber gauge panel kit with a full set of New Autometer gauges for sale. Cost $1029 new, so i'll start the price at $700 shipped and go from there. Gauge part numbers can be seen in the pics.
I would pull the front carrier out, break it down, and clean as much as you can. Throw in new inner axle seals and you'll likely find that the outers need some attention. Go ahead and order spindle bearing and seal kits and some hub seals. I sure the wheel bearings need some attention too...
One of my "dream" trucks would be a lowered K5 with an AWD drivetrain and for it to be powered by an 800+ hp big block. I just think that would be a one-of-a-kind badass ride.