Is it safe to assume your temperature reading is accurate?The water pump is currently a Weiand. I tried an Edelbrock and a stock pump. The Edelbrock water pump on my sbc used an impeller that resembles a turbo impeller, but the Edelbrock impeller for the BBC looked like a stock pump impeller. But, all 3 pumps performed the same on my car.
The timing was 26 initial and 36 total mechanical, with 10degrees in the vacuum can. It is currently locked out at 34. The rear gear is 3.73. You are correct on the radiator, the flow is side to side. The dual pullers are not shrouded, and I tried removing one to see if the fans were impeading air flow and did not change.
I put an air dam below the core support and the slowed it down, but temps still keep climbing.
3.73 is not a horrible rear gear ratio but it hurts on the highway compared to a car like mine with an OD. 2200 rpm at 75mph.
I definitely recommend you put the vacuum advance back on, the engine will run a little cooler on the highway at moderate cruise with closer to 48-50 deg advance. That might be all the difference you need if you're on the edge.
Check if the lower water hose is internally supported with a spring. With the car hot after a drive, hood open, bring the rpms up to 3000-4000 rpm. Watch the lower water hose to see if it collapses. I would try this even if you know it has a spring just to make sure it's doing it's job.
Do you have a thermostat and if so, what temperature and brand?