273 Overheating

Sadly it wasn't the water pump.

Still overheating. The water pump I pulled off looked fairly new and there was nothing wrong with it (Standard / Non-AC). I had a brand new AC / Heavy Duty Water Pump here for one of my 340's, so I installed that on the truck. Thinking maybe I needed something a little more than a standard pump

Still wants to overheat.

Radiator Cap looked kinda ancient, so I put a new one on. No difference

Pulled the New Thermostat out, to see if maybe it had gone bad already, nope, no difference with it out either.

While I had the water pump off, I took a peek inside the cooling passages, clean. Knocked some of the freeze plugs out and checked inside the block, clean. Replaced the freeze plugs I removed, filled the system with coolant. It's circulating, (exactly the same as it was with the Standard duty water pump), but I've noticed it doesn't circulate nowhere near as well as my 6-71 Blown 340 Duster does in comparison

On the D50, coolant moves slowly across the radiator as the engine is running. On the Duster, it's moving fast enough to look like the pump outlet on a swimming pool. (and the Duster runs about 185* all the time).

Hmm, maybe something is plugging up a cooling passage somewhere? I flushed the system again. No change.

I've run out of things that could possibly be wrong.