Thanks for all the t-stat experiences! I'll definitely try replacing it with one of the recommended brands before I change out the radiator. A modern aluminum one would be nice but I don't plan to spend $1000 regardless...

More data - today was low 80's. With the fans on, idling and stationary, it will stay on the 'stat (182-185). Driving at 25 or 55 mph, temp creeps up until the fans kick on (198), then it comes back down to 192 and the fans turn off. If I keep the fans locked on, it will stay at 192 regardless of speed, and eventually drops to the stat temp if I stop and idle for a while. On the hot days (90's) I don't think the fans ever shut off, since the temp would not go back down to 192 while driving.

So either not enough radiator, or (hopefully!) too restrictive a thermostat. Will see what I can find locally (probably have to mail-order it as usual).