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Thanks,The truck looks like quite the beast. As far as the heat, The heater core has to have hot water flowing through it AND air flow across it. These trucks are notorious for plugged A/C evaporators since there are no cab air filters. I see your A/C is not connected but the evaporator is the first thing for air to pass through and condensation usually combines with dust to create mud. Chances are it's pretty plugged up. If you're not going to run A/C just remove the core.
As far as the heater core plugging, it's usually from the inside due to rust and crud from the cooling system. I've had good luck flushing them out with phosphoric acid. I did it on the bench when I had the heater and A/C box out for rebuild. I think you can buy a new core these days too. There are two small O-rings needed for the heater core. They are for two tubes clamped to the core that go through the firewall.
If you have the box out to replace it you may as well replace all the old degrading foam seals and make sure your doors/flaps work. AND don't forget to check the heater valve in the engine bay.