How will the FASS overflow the one tank? It doesn't on single tank trucks, so why would it on dual tank trucks. You hook the lift pump up like factory so it pulls fuel from the front tank, and returns it to the front tank. You can't pump more into the tank than there is for the FASS to suck out. The ECM only engages the transfer pump once it sees the level in the front tank dropping, and stops the transfer pump once it sees the leve; reach a certain point(turns it on at roughly 75% front tank volume, and will shut it off if it goes back above say 80% or so). Hook your FASS up to the front tank to ONLY pull from the front tank like every other single tank install. Run your return to the front tank just like every other single tank truck. Hook the transfer pump back up, and let the ECM do it's job, otherwise deal with the fuel gauge on empty as it is doing what it is designed to do(the ECM defaults to empty ANYTIME something else causes fuel to be trasferred to the front tank. Refueling with the truck running can also cause the gauge to go to empty and require a restart for the gauge to come back).
