It’s certainly possible to modify a tank coffin, but it would probably be more work than removing it and adding under deck bulkheads to create a new coffin. Even that might not work for the Polly tank, since the stringers get wider at the bottom anyway.
As others have said, the best option is probably to get an aluminum tank welded up for you. It’s not cheap, but in the end, it’s the simplest best rout. I would be reluctant to patch a corroded tank. JB weld would be at best a stopgap. If you have some visible corrosion, then there is likely more that you can’t see. It only takes some vapors and a spark.
