In regards to a control valve to regulate hot water in the heater, other links have indicated that there was a shut off valve installed at the point the heater hose attached to the engine block (the hose carrying the hot water to the heater). You can see this valve in some of the old engine pictures, and several of the aftermarket companies (Chevy Duty for example) carry the valve. However, it is just a simple on/off valve and you can get something workable from a hardware store.
Apparently it was common to turn the valve off in the summer and back on in the winter, but someone pointed out that letting radiator fluid sit uncirculated in the heater core for an extended period of time could cause problems.
Dave
