Drill a small hole (3/32") in the thermostat which will allow any air to pass through the hole and allow water to contact the thermostat. The hole needs to be pretty close to the 'seat' of the thermostat. If you put the thermostat in boiling water you will be able to see how it opens and where the hole should be placed.
Also, if the head came off of a 235, you might need to add steam holes in the head. In a 261, the cylinders are siamesed and there are 1/4" holes near where the cylinders meet in the block. Use the head gasket as a pattern and drill steam holes in the head and you should be good to go.
