The Chevrolet and GMC dealer installed recirculator heater was much different in 1947-48. In 1946 and older plus in 1949 through 1957, they sold the traditional round core design but for 1947 and 1948 it was all different.
The attached photos show the 1947-48 GM recirculator heater. Its rectangular core and vertical mounting studs are reserved for just these two years. Except for the logo plate they are the same for GMC and Chevrolet. To be sure the dealers mechanic installed these accessory heaters correctly, holes were placed in the firewall during the trucks construction.
In this photo of a 1948 firewall, arrows point to the factory holes to make sure the heater is installed just right.
An additional point of interest on this 1947-48 heater:
The defroster appears to be an extra cost item. Note the picture of the truck with the side mounted defroster. Also see the separate heater with a round factory plate covering the defroster position. It appears you could order a style of recirculator heater depending on the climate in your area.