Sorry it took so long for me to reply.
The problem with capacitive type probes is that they require a grounding rod or metallic plate to work properly. Most of the underground tanks installed at service stations where I live are now fiberglas or similar composite construction. The older steel ones had the unwanted tendency to leak after a few years.
Use of a magnetic float may not be the best solution, but it's worth looking at. Custody transfer is usually a PD meter built into the fuel dispenser. Tank level is only an indication of supply availability.