Bobby,
Here are some photos of the heater for the 1946 Chevy. You will need two ducts under the dash (they fit in the slits at the top of the dash) for the windshield defroster to work.
Be aware that many types of heaters were used in these trucks-whatever could be found, but the heater below was the only one listed in the Salesman's Handbook.
Harold
1946 Heater 
