For the narrow down a DS cowl vent being present will help, 47-50 have it 51 on does not. The next thing to check is the serial number tag on the DS between the hinges.
GMC will be like this, again year dependent.
https://www.flickr.com/photos/advance-design_parts_co/8130478934/in/album-72157631870734114/
Chevrolet will look like one of these, again year dependent.
https://www.flickr.com/photos/advance-design_parts_co/15837520535/in/album-72157631870734114/
https://www.flickr.com/photos/advance-design_parts_co/8130552154/in/album-72157631870734114/
Other than the radio grill as mentioned there is really no difference, I have seen this plate on the floorboard of some GMC cabs. I am unsure of the years equipped but it may be part of the puzzle.
https://www.flickr.com/photos/advance-design_parts_co/13748509423/in/album-72157631868452754/
This is the only Chevrolet I found the plate on (1 in 32, I owned) so it may be early series or plant specific. You can see the oblong hole it covers near the column. This truck is a 1947.
https://www.flickr.com/photos/advance-design_parts_co/8593171495/in/album-72157631870734114/

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr