I spent the day updating the document below to include the DC Motor Controller 551stseries was so gracious as to share with us.

I tested it on the bench and it looks like a very solid way to go however there is much more to this conversation than that! I have a larger potentiometer on order with an OFF Detent, and an aftermarket rotary switch (the old 6v kind for looks) that I plan on tearing down and using the knob and bracket so it looks decent for our purposes. Lots to it, but it's mostly all here:

http://devestechnet.com/Home/Native12VUpgrade#heatersystem

Scroll down until you get to it. This controller works for both 6 and 12v motors if installed correctly! (Thanks Wayne)

Deve

1950 Chevy 3100 Deluxe Cab
1950 Chevy 3100 Standard Cab
In the Stovebolt Gallery
The Think Tank
More info and tips at Deve's Technet