. . . When you install the 261 on an AD, to do it right you get Dave's water pump adapter plate, then order a 1953 water pump. . .
Why a 1953 water pump?
The 1953-54 water pumps had a 3/8' belt.
Earlier water pumps had wider belts.
Use the earlier (1955-1st or earlier) water pump that matches the belt-width you need for your Advance-Design (or earlier) truck.
Why are you discussing/bring-up the water pump here? I thought this post started with a question about the 261 thermostat housing (on a 1959 engine)?
Regarding the thermostat housing:
I would not use the 1959 261 thermostat housing on a 261 being installed in a small truck. I'd use a 235 thermostat housing so that there would be no need for a hose-diameter adapter. The 1959 261 (the engine being discussed) was used in trucks with "big-truck" radiator hoses (the radiator to water pump hose and the radiator to thermostat housing hose were larger diameter than the hoses used in small trucks).
