The correct GM heater for your truck looks like these two as I believe the square version was released some time during the war years. It was a carryover into the AD years until the stock ran out. The version shown in the 1941 filmstrip is the same as shown in the 1946 Salesman Data Book.
https://www.flickr.com/photos/advance-design_parts_co/15674350367/in/album-72157631868452754/https://www.flickr.com/photos/advance-design_parts_co/23308088454/in/dateposted-public/https://www.flickr.com/photos/advance-design_parts_co/9201965046/stats/This shows the heater with the Special Deluxe version defroster unit, the square later unit has a similar set-up attached to the side (picture is from 1947/48 Salesman Data Book).
Here is the standard unit as advertised in a 1941 filmstrip.
https://www.flickr.com/photos/advance-design_parts_co/23875945145/in/album-72157662039967980/Here is how the unit looks NOS and how the dealer or customer would have received it.
https://www.flickr.com/photos/advance-design_parts_co/8129307001/in/album-72157631868452754/Here is the NOS defroster unit (car version) that would be needed.
https://www.flickr.com/photos/advance-design_parts_co/13978113599/in/album-72157631868452754/Here is the GMC unit (early) love this one.
https://www.flickr.com/photos/advance-design_parts_co/23853563311/in/album-72157631868452754/Now for aftermarket there were a lot of heaters manufactured for trucks and cars. Sears, Montgomery-Ward, Firestone, BF Goodrich, Ha Dees, Arvin, etc., here is a BF Goodrich example.
https://www.flickr.com/photos/advance-design_parts_co/8377831744/in/album-72157631868452754/Here is one from Ha Dees
https://www.flickr.com/photos/advance-design_parts_co/16210861552/in/album-72157631868452754/Finally here is one from Arvin (many different models available).
https://www.flickr.com/photos/advance-design_parts_co/15795508923/in/album-72157631868452754/Expect to pay $150-200 for the OEM type, widely available as they were made over several years and for both cars and trucks. The aftermarket are $200-300 for good NOS with all the parts as shown, if you buy one I would try to get the most parts you can, switches and ducts can be expensive all on their own. I recently saw a Delco-Remy NOS switch like this one go for $150 on eBay, average can be $50-$90. Keep in mind the early two part heater/defroster units use two switches.
https://www.flickr.com/photos/advance-design_parts_co/8129301295/in/album-72157631868452754/
