Hey, Mikey! I can't give you the precise dimensions as the hatch has been repaired. Go check my thread ' deck replacement' and you can see everything under your deck. The 6 x 6 isn't going to access anything. Is there an inspection plate toward the front under the console? If so, you can use a mirror and tape measure and get a close idea where to cut a new 6" round access plate. The sending unit will probably be near the middle/rear of the tank if it's original. Best I can do is show you the pic of the hatch removed, maybe you can get an idea by scaling it. Good luck!
http://www.classicseacraft.com/commu...1&d=1550577922