Flathead cooling advice wanted

[JWL]

I doubt if adding thermostats will correct the root problem. I am wondering how you decided the problem occurs during deceleration?

The first thing I would check is the sealing of the radiator cap. If the temp does not exceed 210 there should not be puking. There are dozens of other potential causes for puking caused by overheating. These range from improper ignition timing, to inadequate radiator air or water flow, to combustion gas leakage, to dragging brakes, etc. etc. etc. I think you will have to more thoroughly examine and describe the problem before a definitive response can be offered.