[QUOTE=Guenter] and, by the way, can the themostat just be 'yanked' out? or does it need to screw/twist or unbolt in some way? I guess it is just in really tight because of old age

Once you've unbolted the three 10mm bolts, the thermostat housing just lifts straight off. You may need to lever it a bit to unseat it from the rubber gasket though.

Dig out the black o-ring with a small pick (this is the important part), then the thermostat will come right out.