I know how thermostats work, thanks The last time I built a big-block A-body was in the mid-'80s... Engine was a mild 383, but only a 22" 3-row radiator, flex fan (and no shroud either). And the gauge stayed on 180 like it was nailed there.

Will have to experiment more on the highway, but when coming down from 65 mph with the throttle closed, there is a slow temp drop but not a dramatic plunge.

Switch on the other side of the 'stat doesn't hurt anything since the thermostat is always open (the fans turn off at 185). (If the turn-off point was below 180 the fans would never turn off, or at least take a long time to do so with the 'stat closed. The gauge sensor is in the water pump at the usual port. There just isn't room to put the switch in the pump, but having it in the upper hose is preferable to those push-in-the-fins ones. A bung in the top tank would also work, of course. And that's only about 6" from the current location...

Don't know what brand of (aluminum) housing I have, but I did stick my fingers in the ports to make sure it wasn't an early 440source housing with the known restriction