Are you talking about the arctic top and doors, or a fuel fired heater?
There were two variants of heater, Swingfire and something else. They are in the TM. My understanding is you can't even start it up unless its like -30 out because it will turn the cab into an oven.
If you can find a complete kit it shouldn't be too hard. You have to add a second pickup to the fuel tank, though there is a bung for it.
The arctic top and doors go on the same way the regular ones come off. Takes maybe 30 min to do. They just have extra insulation.
Augi