In 1955 Blue Chip GMC 630 trucks, the standard electrical system was still 6v positive ground, except for Diesel engines, which were 12v positive ground, with a 6v companion battery. 12v was an option for non-Diesel 630s in 1955.
http://www.1955gmc.com/1955-59gmcliterature/pictures/1955NewModels.pdfMost likely, all Z-series (54/55 New Design) GMC trucks had 6v postive ground electrical systems? I'll try to remember to look at a Shop Manual.
A different world from the 6v negative ground 1954 chevrolet (same for all size trucks) that started this discussion.
