I put a rear tank in my 6.9. It never had one since I bought it. Had to also get a new tank selector valve. I got an FSV2A and wired it to the toggle switch that came with it. Seems to be working, I filled both tanks and have to wait to see which one is really running. The switch does change the fuel sending unit as well as the tank feed, and the gauge responded right from both tanks when I tested it with the new tank empty, now it's just a matter of seeing if it is pulling fuel correctly with which gauge it's reading. Might have to swap wires around.
Also, when I put the tank in I tried a "different" fast fill mod of pulling the plastic check valve thing out of the top of the tank and using a grommet with a 3/4npt to hose barb connecting with a fuel hose running upwards into the frame. Never have I seen fuel go into the tank so easily.