Good point Cab, I can't recall if I checked the mechanical gauge against a pot of boiling water or a mercury lab thermometer.
But the old and the new stat opens right about the temp it's supposed to(183-184F), so I can tell it's pretty close.

I put in the Milodon high-flow thermostat today. It looks just like the 'stat that was in there (and the old one has a Pentastar logo on it, too). Didn't make any difference

So the next move would be a new aluminum radiator since rodding or recoring the copper/brass one would likely be more expensive than buying a modern one.
Or I can just live with the slightly-higher-than-I-like highway temps, as previously pointed out