My 1972 GMC has the holes in the bed and on the upper front fleetside on both side of the truck from Aux tanks. The holes for the selector switch are still in the floor right below the drivers side of the seat. One of the PO's removed them.
Look at the front of the bedside just behind the stock filler neck in this pic. You'll see the hole where the aux filler neck should come through. There is an identical one on the other side.
Gary