The tach is the big stickler. The topic of six cylinder tachs has come up before with the only very pricey 54 Corvette tach being one of the few offered. I think there was mention of some construction machinery tachs or maybe boat and airplane ones. Many were cable driven back then and if your GMC is still positive ground then an electric one may be extinct. Oldsmobiles in the 60's had what was called a "Tic-Toc-Tach" which was a combo clock and tach but it was 12v negative ground.

Evan