Call Russ Eyres. He will clarify among other things, the floorboard rule in the turtle deck.
About 20 years ago yrs truly built an H20 tank for a '29 that pretty much filled up the entire trunk area similiar to what was suggested above. We did do some trick things internally before it was welded up, however, to keep sloshing around to a minimum.
Never heard anything negative about the car or installation.