The heaters in those trucks are pretty simple.. it's really just a small radiator with an electric fan. Hot coolant flows from the engine, thru that little radiator, then back into the engine. The electric fan blows air thru that little radiator, which warms the air.
Not much needs to happen to get lots of hot air out of the heater, so the question is, whats NOT happening?
A dead or dying electric fan will not spin well.. is the fan spinning as fast as it used to?
Dust and dirt on the heater coil will prevent airflow.. it'll feel like the fan speed is set to low even when it's on "high".
Clogged hoses or coil will prevent hot coolant from heating the heater coil, which results in luke warm air comming from the heater.
It's my experience that a properly functioning stock heater does a great job of keeping the cab warm.. in fact, I put a '50's heater in my '46 1/2 ton and it works like a champ.
