I believe the GMC's all used Sun tachometers, which in that time period used a separate transmitter box. As long as the box was labeled for 12 volts and 6 cylinders it would work fine. The tach didn't care if the engine was inline or a V configuration. As noted above I don't know if the dash layouts are compatible, however early Sun tachometers were assembled with screws and nuts. If the layout requires just the tachometer mechanism, and the V6 version is in fact a Sun model, the mechanism can be easily removed and adapted to the other dash installation with a little creative engineering.
