Are there any OM636 experts out there?

[DAR2KMR2]

I have a 1958 180D that sat for about 30 years .... Over the past 3 years or so I've been working to get it back on the road. I got it running and stopping fine .... but it quickly developed leaks out of the freeze plugs. In addition to the leaking coolant the coolant temp gauge quit working and the engine over heated when it ran out of coolant. I got the leaks fixed and put everything back together and it runs fine once again. However, now with the external thermostat in the radiator hose in place, the engine heats up and pushes coolant out the overflow pipe until it runs low and overheats. With the thermostat replaced with a straight hose the coolant circulates to the radiator and there is no leakage. The thermostat has been replaced with a new unit, installed a new water pump, and flushed the cooling system twice. I thought that perhaps when it overheated the head gasket was damaged or the head cracked causing higher than normal pressures in the cooling system; however, there is no oil in the coolant and no coolant in the oil so I'm thinking the head gasket is ok. Any thoughts of what is happening?

Thanks.