350 Won't Stop Overheating

[5Tractorguy]

Have an overheating problem with the rebuilt 350 in my 1-ton. I'm at wits end trying to figure it out, has to be something simple. Hoping someone here may have an answer...

350, bored .030 over with a Melling MTC-1 Cam and 333882 heads
New stock water pump
New 180-deg thermostat (this is the 3rd thermostat this week we tried).
New thermostat elbow
New radiator hoses top and bottom
Radiator was new about 6-8yrs ago (has been flushed during all this, clean as a whistle, even looking in with a flashlight, no buildup).

- I was able to run the engine for the cam break-in without the thermostat, the pump is pushing coolant, and the engine did stay cool for the break-in run.

- But as soon as I installed the thermostat, it overheats... every time. The pass side of the rad is getting hot (where the heater core cycles through) and the drivers side is cool.

- Confirmed that the thermostat is opening when hot by removing the rad cap, blowing back through the top rad hose towards engine and having the coolant rise/spill over rad fill.

- The radiator fins are clean, the fan is spinning, belts are tight, fan clutch is good, rad does have a fan shroud.

- I've tried leaving the rad cap off to purge it, rigging up a bypass line from pump to intake manifold, pinching off the heater core, pulling different intake plugs or hoses to burp it, etc.

So before I go buy a new radiator, or do something drastic like pull the engine apart... anyone got any ideas?