I drilled a 3/16" hole in the floor close to the fiberglass support, just ahead of the kick panel. This puts the wire just inside the stringer, and its convenient to wire to the gauge. Longer length of wire should not adversely affect the gauge reading, but don't expect any real accuracy due to the float and fuel level line in the tank. I've found that when my gauge becomes motionless on "E" I have about 8 gallons left. I could pull the sender and add a longer rod to the shaft, but not sure its worth the effort. I'd really like to add one of the fuel counters/meters, Faria had one, but I cant seem to locate one, and my dash is somewhat full now. New project is likely to be FI, so I can pull that data from the engine CPU.
Your project is looking gorgeous, hope you get it out on the water soon.