jb, the thermostat from Wattman is the same diameter as OEM but extends farther down into the housing.
you will find that there is a 'ledge' inside the thermostat housing
that ledge is just large enough to keep the thermostat from seating all the way. I milled some of the ledge away and the new 'stat dropped right in.
I have a spare thermostat housing and could easily mill it out for you if you decide to make the switch. As for getting a drop in thermostat...you'll have to do your own research there.
Peter Y.