One other possibility is a slightly clogged heater core. At some point might flush it out. It can be clogged and still let some coolant to flow through. Just not enough to quite heat the cab.
My 58 never has heated the cab very well. Even when I was small and my grand dad drove it, in the winter it was a cold truck.
I have had the heater core redone many years ago, and still does not move a lot of heat.
Before the next winter I will be installing a new heating and a/c system so maybe I can finally get the beast warm. I run a 195 thermostat year round in my truck.
I drove a 1960 48 passenger Chevrolet school bus with the 261 in it, and in the winter time the engine could not generate enough heat with the inside heaters running.
