I am installing an original 6v GMC radio from a 49-52 pick up into a 52 Chevy pick-up. Have changed the dial faceplate to show Chevrolet, but wondering if the original GMC radio was positive or negative ground or doesn't it matter?

Yes it does matter. If the GMC has had recent work it will have solid state components that have a life expectancy of 1 nanosecond if connected incorrectly. The GMC is positive ground.

Well, this was an e-bay purchase, and it was supposed to be "restored" although looking inside the radio, it appears all of the old components are in there, and they were simply marked "OK" on each piece, which leads me to believe everything was just tested and is all the original stuff. Looks like exterior plastic pieces were replaced and the housing painted, but that may be it. Having said that, my son did connect the radio in the negative ground truck, and it worked fine, although was only getting one channel in the shop. So, if it works, are we ok? Anyway to tell on the outside of the radio if it is negative or positive ground?

No! If it has been restored it will most likely have a solid state vibrator. Note the instructions on the side of the vibrator. It will give the voltage and which side is ground.

If it has a mechanical vibrator it does not make any difference on the position of ground.