It is my understanding that GMC went from 6v pos.gnd to 6v neg.gnd in '54, and at the same time changed to the round solo gauges instead of the cluster gauge. The '55-TF series was 12v neg.gnd, and despite the fact that they look identical the fuel gauge would have to be altered from the factory for 12v. I doubt you'll need to add a reducer if they're TF gauges.
IIRC, the (+) post is behind the E; the tank sender connects to the post behind F. I trust someone will jump in with corrections if I'm backward (again).
