[JSeery]

For starters I would disagree with your radiator guy. The hotter you run the engine the higher the efficiency, the better the lubrication works and the less sludge buildup you will have. I would want to run at least 180 degree thermostats. The thermostats do not determine the maximum temperature your engine will run at. The increase the rate at which your engine gets up to operating temperature and maintain a minimum operating temperature.

The original thermostats for the early flatheads was a barrel shape that fit inside the radiator hoses. You can use modern style thermostats but they require a method of retaining them in position. There are numerous threads on different retention methods.