The OEM fuel tank for a Boston Whaler 1972 OUTRAGE 21 boat may have been fabricated by
Florida Marine Tanks. I recommend you contact them to confirm they were the OEM. If Florida Marine Tanks were the OEM, they can probably fabricate a new tank for use as a replacement.
Obtaining a detailed drawing of a fuel tank originally made by Florida Marine Tanks from them at no charge and sufficiently detailed to permit a new tank to be made by someone else will probably not be likely to occur.
If Florida Marine Tanks is not the original fabricator, the best method to find a replacement tank will be to carefully remove the OEM tank from the boat, and use the OEM tank as a model or pattern for the fabrication of a custom replacement tank. Without knowing your location, the existence of a custom aluminum welded fuel tank fabricator in your area will have to be left to you to discover.
The OEM tank can also be carefully measured, and an accurate mechanical drawing of the tank can be made from the OEM tank by someone skilled in mechanical drawing and fabrication of objects from a drawing.