[SkunkBoat]

About V20 foamed-in fuel tanks...

The fuel tank space was a sealed compartment. There was a pvc pipe to allow water to pass from fwd to aft bilge. The deck hatch was meant to be sealed watertight. Any water that got into that space stayed there.

You have to decide if you are going to make it a bilge space or seal it.

If you foam it you have to make it sealed with a pvc pipe. Foaming has the advantage of easily securing the tank without brackets. Of course, we know the problem of foam and aluminum tanks.
I foamed in a new aluminum tank because(at the time) I could not find a correct plastic tank with fittings in the same place and workable dimensions. I do believe there is a 55 gal Moeller tank out there that is correct.
I think a foamed plastic tank in a sealed compartment is the best solution.

oh, be careful if you find a plastic tank. New design tanks are not vented per EPA rules. You need a vented tank ...or you need to figure out what extra parts are required to use a non-vented tank.
Also, I worry that non-vented tanks expand and contract quite a bit so foaming may be a problem.

If you go with an open bilge space you will have standing bilge water under the tank and often touching the tank. You will have to hang the tank from your new stringers with bolts.(drill holes!!!)that's 360+ pounds

As for foam in the space outside the stringers I think it will deaden the sound and make it feel much more solid. Same with the foamed tank. Of course, it will be safer also.

Oh, plan on installing a forward bilge pump. You have the opportunity to plumb the hose out the side!