Truck Repair: Rebuilt /New crate motor running Hot, fan clutch, 1 ton trucks

Question
Van,
  I have an 84 GMC K1500 4X4 Pickup w/ 5.0L/700R4 and AC.  I rebuilt a 350 I had on hand with all new components and all clearances in tolerance and had this same problem, the motor would overheat after running for about 3-4 mins or 1-2 miles.  I tried several things to no avail and pulled the motor thinking maybe I missed something on the rebuild.  I installed a new 350 crate motor from GM, and am having the same problem.  I have double checked the timing, rebuilt the carb (which is one year old), installed a new distrubitor,new radiator,tried two new waterpumps, two thermostats, three different temp senders, and just installed a new exhaust(Y-pipes TO Tailpipes with new cat converter/305-350 use the same componets).  All tune up parts etc. are new also, this is why I'm starting to question my mechanical ability.  Will be trying a new radiator cap and fan clutch tomorrow, they're the last cooling system parts I can think of that have yet to be replaced.  I live in New Jersey a little above sea level so air pressure is concistant, I have basically ran out of ideas at this point so any ideas form you would be helpful.  It's basically a "new" truck and hope to get it right.  Thank you for any help.

Answer
Hi Mike,
The fan clutch isn't the problem if it overheats while driving down the highway, since a fan isn't even needed then. You can drive all day without a fan if you have air flowing from road speed.
There is one thing you talked all around, but didn't mention.
There is a vacuum operated heat riser valve in the right exhaust manifold. It is operated by a temperature vacuum switch, which allows vacuum to flow and close that valve while the engine is cold, forcing the exhaust from the right bank of cylinders to flow thru a port in the intake manifold, and out the left side.
I had four 1985 1 Ton trucks with that engine, and every single temp vacuum switch failed. Every one of them would overheat.
If you disconnect the hose to that heat riser valve, it will spring open. Vacuum closes it.
I still have one of those trucks, and it hasn't been connected for about 12 years.
Hope that's what you find.
Another possibility is the transmission overheating, but I feel you would have found that.
Van