For what it is worth, I have a '54 1/2 ton, and I recovered my seat (still have the seat back to do) with Jim Carter's Spanish Grain Brown kit. My truck had some of the original covering left on it, and it is a pretty decent match. I'd post a picture, but I never seem to have my camera handy when I work on the truck. I can't speak to your '51 GMC.. so hope this helps.