Steve, based on the air flow test and the 3 degree drop in water temp in your radiator, you are not making enough heat transfer from water to air. I checked my 71 RR last Sunday and had an 18 degree drop (coolant) across the radiator and 160 at the thermostat (housing). My 71 RR has a 22" radiator with 5 blade clutch fan.

Check the air temp entering and leaving the radiator. I would think there should be about 15-20 degrees difference. In 2005 I checked my Challenger (with a 440 and A/C) and had an air temp increase across the radiator of 30 degrees and a lower hose temp of 157. The Challenger had a 26" 4-row radiator.

I bet you are not getting close to that. Bottom line the tubes in the radiator may need to be cleaned to transfer heat properly.
Ron