273 Overheating

It's not a Duster (pretend it is), it's my D50 4x4.

Specs : 273 Small Block, Bored .060 Over with Forged Dome Pistons, .500 Mechanical Lift Cam (manufacturer unknown), 340 J (1.88 valve) Heads, 10.1:1 Compression, 340 Spreadbore Intake, 800 Thermoquad carb, 1971+ LA Timing Cover, Balancer & Water Pump. 22" A-Body Radiator, External Transmission Cooler, Mechanical Fan + Electric Pusher Fan. Mechanical
Gauges.

It didn't used to overheat, when I had a badly leaking 26" C-Body Radiator in the truck. Swapped in a like new (Spotless inside, not even a speck of dirt) 22" A-Body Radiator and ever since it's ran hot. Coincidence? Probably

We're talking 230+ degrees and still climbing.

First thing that came to mind was the Thermostat, I pulled it out and sure enough it was stuck shut. Didn't have another one on hand, so I put the water neck back on without a T-Stat to see if it made a difference. Nope, still overheats (odd, I thought for sure the T-Stat being junk was the cause). Pulled the Radiator Cap to check for circulation, coolant is circulating through the radiator.

Ok, maybe it's circulating too fast without the T-stat and doesn't have a chance to cool down. Put a new (tested in a pan on the stove) T-Stat in, functioning, still overheats.

Ok maybe it's Airflow? Wait, the mechanical fan is drawing enough air to spin the electric fan full speed (without it turned on), and it doesn't matter if it's idling or running 65mph it still overheats. At 65mph it wouldn't matter if the fan was drawing enough air. So it's not an airflow issue.

Maybe it's the timing? I was screwing with the timing a while back, hoping it was a tuning issue. So I grabbed the timing light and tried setting the timing in various places. Tried Everything from 28* to 34* at Full Advance (3k rpm, give or take 100-200 rpm) with the Vacuum Advance Canister disconnected. No Improvement, it still overheats.

Maybe it popped a head gasket? No Oil in the Coolant, No Coolant in the Oil, it's not consuming coolant. Pressurized the cooling system to 20psi and left it sit for 2 days, it held pressure the entire time. Ok, maybe a Leakdown test. No, it's not showing any problems there either. (I still have to run a compression test as well)

Maybe it's crud in the cooling system? Flushed the whole system, 3 times, saw nothing but clean flush come out.

Other than overheating (idling or driving), the truck runs perfectly. And it didn't used to overheat. It used to run 190*, no matter what, except for when I would tow with it, then it would get up around 205*

Any ideas on why it would start overheating now?