Thermostat or No Thermostat?

how sure are you the rad is clear? I'd check temp with an infrared gun, or if you can check it with your hand, make sure the rad has an even gradient of heat/cool on the fin surface. If you have isolated cool spots where they shouldn't be, it is indicative of plugged tubes, very common problem.
Also, I didn't see mention of rear gear. I never had an overheating issue until I went from 3.9's to 4.30's.
One more thing, removing the thermostat is not a solution.