It just makes good sense to have individual gauges for each tank, instead of having to rely on the temperamental tank-selector switch to know where each tank stands.
A second-best alternative, and the way I have my factory fuel-gauge, is to eliminate the gauge circuit from the FSV (fuel selector valve) and route the THREE wires through a SPDT switch, with the wire that feeds the dash-gauge being on the center terminal and the tank wires being on the other two terminals.
I have a separate independent SunPro gauge for the big auxilliary tank.
Considering the notoriously poor reliability of the factory sending-units, were I going to install separate after-market gauges anyway, instead of worrying with finding gauges compatible with the crappy factory senders, I would replace the factory senders with standard universal SunPro senders and their matching gauges.
Thus, in the highly un-likely event that one of these SunPro senders were to fail, a very affordable replacement can be had from just about any auto-parts source.
A little bit of quality time spent in hanging and adjusting the float-arm will yield a fuel-level measuring system far more accurate than anything available from Ford.