I used 3003H14 .025 for the belly pan. No need to use .032, but you can if you want. Allclad is kind of a waste on the belly because you're never going to see it.
48" wide strips hit every other crossmember if you install them side to side. I worked my way back to front so the seam faced the rear. The large flange rivets from VTS did a great job.
Support requirements depend on the tanks you buy. I used the gray tanks from VTS, which are required to be fully supported from the bottom, not just strapped. I used steel angle welded/bolted to the frame and 1/2" plywood to support the gray tanks. I used 3/4" plywood for the fresh tank because it had to span a larger area. Lots of detail and photos on the blog in my signature.