I pulled the thermostat out and tested it and it seems to work just fine. It is an original Mopar thermostat, even has a part number on it. Not sure how old it is, could be 20 years or more. No idea when I bought it or why. I used some boiling water to open up the thermostat and then gradually poured cold water in to see when it closed. Looks like it starts to open around 160 and is fully open at 180. I have a thermocouple attachment for my voltmeter which seems to be pretty accurate. I'm still going to put a new thermostat in there but I'm starting to suspect that this setup just runs cool. I'll know more once I get the engine running with the new thermostat.