Update for history, posterity, whatever:
Did a vacuum leak test. Used just the acetylene from a torch set-up. Hand-held propane didn't raise the rpms when held at the airhorn. Acetylene worked perfectly. Very obvious rise in rpm when held above carb. Good news is, no vacuum leaks in evidence anywhere.
Warmed up the engine and readjusted the valves. Only helped running a little. Still couldn't get the idle speed down or smooth running.
Swapped to a 180 degree stat. Probably not the culprit, but I wanted it to run at 180, not 160.
REBUILT A SPARE CARTER W-1 CARB I had, using a kit from The Carburetor shop, CarbKing's place. He was very helpful on the phone. Put the new carb on, truck started up with very little choke, ran smooth at high idle, and got the idle down to 500 rpm or so, with NO CHOKE!!!
Runs VERY smoothly, temp pegged at 178, doesn't hesitate when I stomp on it. HUGE difference. This time, anyway, it seems the carb WAS the culprit!
Bought two kits, may rebuild the original carb, see if I can figure out why it wasn't happy. Float level was close-enough. I suspect a plugged idle circuit somewhere, maybe a clogged "equalization" line (tube at top of airhorn ).
Thanks to everyone for the support!
-Michael
