Chevrolet and GMC cabs are basically the same during 1955-1959, however their dash boards differ. Thus, removable dash items such as guages, glove box doors, and radios will not interchange with Chevrolet. It seems it was a way GM divided the two marques using limited expense.
A major difference on the GMC dash is the long horizontal ridge at the lower edge door to door. Therefore, as the accessory fresh air heater was mostly the same in Chevy and GMC, the dash mounted control panels were different. GMC’s must have a hump in the lower half to go around this ridge. The two heater control levers must be 7/16 inches longer then Chevrolet. The upper half including the fan switch is the same on both brands.
This control panel on GMC has never been reproduced as fewer of these trucks were sold. Restorers must hunt for restorable originals. The longer levers are available from Jim Carter Truck Parts and a few full stock dealers.