I just bought this truck. This is my first duece and a half and I really don't know much about these so I would sincerely appreciate any help and advice. It's a 1971 Kaiser with a multifuel diesel engine. The first time we took it for a ride I noticed that after about 5 miles the temperature was climbing up towards 220 degrees. We brought it back to the house and checked the coolant and found it to be topped off and hot. However I felt the radiator core starting from the top and working my way down. For about the first 4 inches or so the core was warm but as I moved towards the bottom it gradually got cooler. The bottom area was ice cold. I was under the impression that the only thing that would cause this was a plugged core however in speaking with some repair people about cooling systems in general it was suggested to me that the thermostat may not be opening. This truck has an optional military heater kit mounted in the engine compartment and both hoses to it are hot, and my son mentioned that he had gotten heat at one point when he drove it. so I would think that means that the thermostat is opening. Is that correct? What would be a proper way to troubleshoot this problem? Thanks, Lenny