If the doors are original to the truck, it's a '51 (vent window with turn down door handles). As far as the grill, the GMC grill is pretty much the same up to '53. The turn signals are below the headlamps on GMC, http://www.stovebolt.com/gallery/english_rob_1951_01.htm , in the grill corners on a Chevy,
http://www.stovebolt.com/gallery/mccormick_jim_1951.htm
I hope this helps you out.

Bill Burmeister