Make your cut 1in. outboard of the bilge opening on the starboard side and 3/4in outboard of the opening on the port side. If you want a full fuel cell access panel it will be 63 1/2in long. If you just want to cut a small hole to access the fuel cell outlet then the panel will be 10in long from leading edge of bilge hole forward to center of cross bar support. I hope this is understandable.
Buck
Use caution and set your depth of cut shy as you will be cutting center of stringers and center of cross bar so that your panel edge will be supported.
buckboardjr's post in 19 FT SEA NYMPH GL FUEL TANK was marked as the answer