I did a modification to the tank on the Catalina 27 with what you are describing. There was a kit available from Tempo that could be adjusted to the depth of the tank. It came with complete instructions on how to trim the float arm. There was a hole in the tanks already, this unit dropped in, there was a gasket and it screwed down into place. The "gauge" part was pretty cool, it screwed to the part that went into the tank but it there was a magnetic interface between to two so there wan no actual hole in the sending unit to the gauge. It worked like a champ. I eventually changed it over with yet another Tempo kit and made it read out to a fuel gauge so I could see the fuel level in the cockpit instead of digging under the Q-berth junk, cushions and locker board. I used a similar kit on the Islander to replace the faulty sending unit, I already had a guage in place. I just did a quick look see, it appears that Tempo is now Moeller?? Google "Moeller Marine mechanical fuel sending unit", I see they make them for different tank depths now. I did not look for the universal one I bought.