I successfully repaired my 150 gallon port side tank two years ago. Was able to cut a hole and used a 12" Seabuilt access plate once the repair was completed. Large and accurate round holes can easily be cut with a Malco HC1 available from any HVAC supply. The HC2 will cut holes up to 20". After mopping out the interior, I used MEK to thoroughly remove all residue. I used KREEM fuel tank liner, but there are other brands such as Red-Kote and Northern. Proper ventilation is key. Pressure tested @2 psi for 2 days just to be sure.
Daniel
'89 43' Tradewinds MY