It's a good question. I guess I had a previous rib that had alum tank in the bilge and it lasted only a few years.
The original tank design hangs the tank from a very thick alum deck plate and the tank itself is burly to be able to just hang there. The benefit is that the tank is not sitting in water in the bilge which is what kills aluminum tanks quick.
So to go back to aluminum, the hanging design would have to be even burlier to support 70 gallons, maybe not a good idea. I wonder why the Navy chose 34 gallons for this design. Seems so small.
So to go alum again I need to put the tank in the bilge, maybe supported up a bit on a platform, but basically sitting down there. Would still need to deal with supporting it, strapping it down, and then also still need to have a deck plate.
The ATL bladders can completely fill the space (fully supported) without corrosion problems. They are well baffled, very flexible (alum you are limited by the deck flange size) and a drop in solution, and also light.
Then we have the integral tank solution. The space is perfect for it. Lot of labor and care to get it right. Burly and maximizes volume. Done right it would be really nice.
Not cheap unless I do it myself and that ship has sailed. All the pieces are going in and I need a tank.
So basically, I've done as much prep as I can but now I don't really have time to make this an art project that I do myself.
ATL and Integral are on the table now.