Underbelly project: tanks hang below frame members . . .

You need some Beam clamps and a material to make up the drop of the tanks relating to the frame. Your description as to what you would do with the area beneath the tanks is exactly how I'd handle it. I'd use aluminum too. I'd also attach a piece of flat rubber as a gasket between steel and aluminum mating surfaces.

Most larger cities have a steel and metals distributor. I use Discount Steel , as an example. DS will cut and ship if that's a necessity.