There is a Pertronix 6v system for positive ground (GMC) and negative ground (Chevrolet) 6 cylinder engines. I have been running one in my 6v Suburban for over two years. It used to be listed in their catalog and the part number was posted here a couple of time.

On a 6v system, make sure your battery cables are at least 1 or 1/0 and the starter and engine spin just fine. I now have 2/0 (00) cables for the positive cable (to the starter) and the negative ground cable (to the frame behind the battery box, but some people run a long ground cable from the battery to the bottom attachment bolt for the starter).

http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=2;t=007101
1167N6 1941-1953 216/235 6v negative
1168N6 1954-1962 235/261 6v negative

Tim