I added a second heater to my 110 for the trip to James Bay last February. It was from JC Whitney and was quite cheap. Made by Flex-A-Lite (the engine fan guys) and will melt glass if you let it.

We were in -40 degree weather, and we had the truck up to 78 degrees (F) inside when we couldn't stand it any longer. We wanted to see how hot we could get it... We had to turn it off and open the windows to cool off or risk heat stroke in sub-arctic Canada in February. Try explaining that one...

Now to be fair, I also have the truck well insulated, with foil-faced fiberglass above the headliner, foil-faced rigid foam (1" thick) on the roof sides, and foil-faced rigid foam (1/4") on the rear roof area. No other insulation (aside from the footwell "insulation" from the factory) has been added.

Mike has one of these on his 101 to replace the big, expensive hard-to find and welded to the truck stock heater. it works better, and is smaller with a better fan. Ther is another version with provisions for ducting as well if you want to get fancy.

And yes, it gets so hot because it recycles the warm air in the truck. The stock heater uses fresh air all the time, so the colder it is outside, the colder it is inside. My '71 is stil stock, and the heater is crappy because of this.