When drilling those holes through the firewall, be sure to not mount the heater too high. There should be 2" to 3" between the top of the heater and the bottom of the glove box. If you ever find a mechanical governor, you will need this space in order to mount the in-cab speed control lever beneath the glove box. Also, position the heater so that the four mounting studs and two coolant supply tubes straddle the rectangular pattern created by the four factory well nuts that exist in the firewall. These nuts are used to mount the accessory surge tank for the radiator. You want the hoses to pass on either side of the surge tank and not pierce through it should you ever add that option too.
These guide lines all pertain to the model 60 and 70 series MoPar heaters. The early Model 36 MoPar Deluxe heater, as found on 1946 through approximately 1951 Power-Wagons, was not designed to work in conjunction with the radiator surge tank. It mounts with only two large diameter hollow studs through which pass the supply tubes.
Clint