When I made my RV-3 fuselage tank I deviated a bit from the plans:
Put the vent in the fuel cap.
1/8" NPT hole top rear to sight gauge (plastic line) down to outlet hole. Didn't make hole for and install the mechanical fuel gauge/sender.
That simplified things slightly.
Thoughts on fixing your wing tank: Since the slosh has held up for 450 hours you probably won't need to clean it out, unless you change from AvGas to MoGas, although I might worry about disturbing it around the area where you'll fix the leak.
Unless you'd like the 25 gal extra fuel capacity for cross-county flights, I don't think I'd bother with making/installing a fuselage tank. Remove the wing tank, cover with soap water (or submerge) and pressurize LIGHTLY. Surely finding and fixing the leak will be a lot less work. Of course if removing the tank ruins your paint job, that's a different matter.
Finn