Hi,
I recently fitted a fuel guage to my narrowboat. It has a s shaped inlet pipe so is unable to use s dip stick. I had to have a 1,1/2 inch bsp fitting welded to the tank. Not cheap especially as I can weld myself but I had to pay the marina owner. The guage was a mechanical float type about £70 I think. My only problem is my tank is shallow around 10 inch deep. This means the gearing is very high so the float is unable to rise when filling the tank quickly , however when running the engine the vibrations cause the guage to work properly on the way down.
Hope this helps Martin