I've been interested in doing this and since I went through the trouble of searching, I thought I'd put the info on the Pertronix part # here. It's 1168LS-N6 for the 1950 GMC 6 cyl, 6 volt, negative ground.

Here it is on Amazon:
http://www.amazon.com/Pertronix-116...ie=UTF8&psc=1&smid=ATVPDKIKX0DER