Hi Dan,
I have a big alu radiator 26” with dual fans in my 66 with a SB 302/331.
I have installed a Craig Davies digital (not manual) Part #0444 fan controller. The probe is installed through the radiator fins near the hot inlet hose. Don’t install in the hose as it will snap off due to water turbulence. I had two fail, but no problems since installing through the fins. As the air temp around the probe is about 15 degrees below the coolant temp, you need to set the On temp at around 15 degrees below your thermostat temp. So, with my 180 degree thermostat, I have my fans set to switch On at 161 degrees. This could be the issue with your fans coming on at 205 degrees.
I have also installed a temp only Engine Watchdog. As this takes temp sensor readings from the engine block NOT coolant temp, I have mine set to alarm at 201 degrees. Best investment I’ve ever made. You can also buy these with Oil Temp and fan controller.
Also, with dual fans, I would advise to get your fans wired separately with two circuit breakers. The thinking here is that if one fan trips, the other fan will continue to run. Just some further protection.
Colin