I just finished a 12 volt conversion on my heater. The motors are not really a direct replacements. I wasn't happy with the one I got from Jim Carter so I returned it. The diameter of the motor was too small and the gasket wouldn't seal it where the back of the motor goes through the firewall.

There is a control valve that mounts on the firewall with a control rod going to the bottom of the dash.

The blower motor switch controls the speed of the blower motor.

I had a local radiator shop backflush and check out my heater core before I reassembled the heater.

If you go to this link it shows how the heater is installed.
http://chevy.tocmp.com/accessories/1951am/index.htm

If you're interested in replacing the blower motor with a 12 volt motor start a new post in the electrical bay or pm me and I will go through the details of what motor I used and what I had to do to install it.