Jeff - I did some surfing through web and other resource documents for the 1937 Chevy trucks (and cars) and found that the truck cabs were not designed/equipped for defroster vents nor did they even have GM accessory heaters/defrosters offered like the passenger cars did (shown in this
1937 Chevrolet Accessories Price List ). There where aftermarket heaters/defrosters offered, but they would have required cutting a hole (or holes) in the truck dash to create an opening for the defroster air to reach the windshield.
The 1937 Chevy passenger cars had a single defroster duct port (covered by a snap in sheet metal plate when an accessory defroster wasn't ordered/installed) on the top of the dash in front of the driver and used a 2-piece defroster vent similar to what you have pictured as part of the Defroster accessory kit available for the cars. The right hand piece (feed duct) in your pictures attached to the underside of the dash and the left piece attaches on top of the dash to divert the defroster output against the inner face of the windshield.
I've attached some pictures snagged from various sources to show the original truck dash panel, car dash panel, car dash defroster opening block-off plate and the original style car defroster pieces to help visualize what I've posted.
Not sure you may want to cut up your trucks dash to install what I'm pretty sure is a car accessory (I don't know what your goal is with your truck), but wanted to offer this info in case you may not want to use a non-truck item with your truck.
