Overheating 72 3/4 ton 402

[w6tractor]

OK, I'm stumped. My 1972 Chevy 3/4 ton with 402 has begun overheating. I pull a 24 foot goosneck with a 4-6K tractor, mower on it. I have replaced the water pump, radiator (new NAPA 4 row brass and copper) and installed a 160 degree thermostat. It still goes to 230+ even on a mild day. Doesn't do much better w/o the tractor and trailer. It runs good, no miss or backfire. It will "run-on" after switched off sometimes when it's hot. It has a 400 auto with a 12"x15" trans cooler in front of the radiator on the passenger side and an auxillary electric fan pushing air on the other side. I also added a coolant recovery tank, set the timing to 8 degrees BTC. What am I missing? Could the trans cooler be blocking too much air? You guys are great with diagnosing.