Hmmm... So I had a chance to do some experimenting with the tanks and Princeton converters tonight. The good news is no shorts between the tank and the plates! Measuring the capacitance between the center pin on the BNC connector and the tank skins, I see the following:
Left tank empty: 145pF
Left tank ~5.3gal: 160pF
Left tank ~10.6gal: 172pF
Left tank ~15.9gal: 188pF
Left tank full: 227pF
Right tank empty: 155pF
Right tank ~5.3gal: 170pF
Right tank ~10.6gal: 182pF
Right tank ~15.9gal: 196pF
Right tank full: 233pF
So not too bad. Capacitance varies with fuel level, as I'd expect. Conclusion: Tanks are probably fine.
Then, I calibrated the (right) 2 point Princeton converter: Empty tank, push button, Fill tank, push button. That should be it (?).
Now I fill up the tank, observing the voltage output by the converter into the G3X. All fuel levels lower than ~13 gallons result in 0.1V to the G3X. Greater than 13 gallons, the voltage level grows exponentially to around 4.8V when full. (Blue line is tank capacitance, Red line is voltage output from converter, as read by the G3X):
This does not look right. The converters cannot seem to distinguish fuel levels below around 13 gallons, and therefore the G3X will not be able to, either. It's looking like I might need to contact Princeton for help at this point.