I don’t know if this helps, but I left my swinging arm sender in the tank connected to the VDO panel as a rough backup and fitted an additional WEMA linear resistive sender right in the deepest corner of the tank, connected to the fuel gauge connection on the Volvo MDI. That in turn is connected to the NG network for fuel readings on the MFD and Victron App. You cold also just connect it to the VDO Panel if that’s all you want.
As for length, I used WEMA 350mm Hi resolution probe – S3 Fuel and Water Sender European (0190ohms) / 350 / High resolution [336008] and the HFL adapter right in the corner of the tank. I realised that the probe was just hitting bottom and I added a 5mm spacer and additional rubber gasket on the HFL in order to raise the tip of the probe just clear of the tank bottom. If you don’t wan to faff with that just use a 325mm probe and you wont be far way from the full depth in that location. If you try and use one of the spare apertures already in the tank the depth reduces quickly as you move from the corner.
Note also that some of the tank apertures have 6 evenly spaced holes and some 5 unevenly spaced holes. Most senders are 5 hole. That’s why I ended up using the HFL adapter as you make your own holes and besides it provides a great secure seal. It just means cutting the tank without getting bits in it but it can be done with care.
Dave