New Fuel Tank
I had an aluminum fuel tank in my boat that developed pinholes in the bottom because it sat on wooden stringers that were damp. The combination of water and lack of air circulation caused the accelerated porosity. Luthers Metal Fabrication of Bristol, RI made a new tank using thicker gauge aluminum with similar metal fittings and clear coat epoxy, so the tank should outlast me. It cost about $400, but that's better than being fined by DEM, if your automatic bilge pumps the diesel overboard. They suggested I use 5200 to attach a high tec plastic used as spacers on the bottom to provide adequate air circulation, which I did. However, an over zealous shipwright that helped me tab in the new cradle decided to inject foam around the cradle, which killed the air circulation part of the repair. I wasn't present for that part of the repair.
The biggest problem was getting the old tank out and new one in. I had to use a couple of 4x4 beams over the companionway with a chain and come-along to lift the motor up and into the cabin. That was the only way to get the old tank out without cutting fiberglass. Then I just put in new motor mounts with the reinstall. Hope this helps.