These are the two correct heaters for a 1950 truck if you are looking to go that way. Rebuilt they are both good heaters and will run you out of the small cab.
http://www.flickr.com/photos/advance-design_parts_co/8129448404/in/set-72157631868452754
Advertise in Swap Meet if you want one, good,rebuildable or driver examples go for $75-150. Rebuilt units or NOS on eBay from $150-$600. There are a couple more parts involved with the Air-Flow retrofit. The recirculating heater has basically 6 two block off plates, switch, vents, and the heater. When new they represented a fairly significant investment, you can see the dealer installed price here.
http://www.flickr.com/photos/advance-design_parts_co/8131119579/in/set-72157631872034068