The stock thermostat on a 90 GT 5.0 engine, was rated at 192*. That was for the EFI set up, but even with a carb, 190+ is fine. I would think a running temp you mention, of 165* is actually to low.
Your coolant fan should kick in automatically, at a pre-set temp, to prevent overheating, I wouldn't use the manual switch, unless temps continued to climb up and up, and the fan didn't kick in on it's own. That could happen if the sending unit was not grounded properly, or it was on the fritz.