Here's a couple of measurements - the 40mm screw-in style, and a later model drop in thermostat. I happen to have both...
The tops of the threads on the brass screw-in are just a bit under 39mm, and the diameter of the drop in is about 54mm
The 45mm thermostat is still in place - a 45mm socket arrives today.
50 years from now - how are we going to restore a "classic" microprocessor based modern car?? Alfas - 1980 Spider, 1987 Milano, 1976 Spider