best scrap available and cheap to build pontoons 40 ft Damon camper pontoon boat

:cool:im looking for tanks that would be great to use (rugged and sealed for chambers)and cheap to build (40 ft x 24in) at least if not bigger pontoons for my water camper  plus my cross framing and decking I'm going to take the rear end out of it and front end and weld it down to 5 cross beams going out to pontoons it will be 16 total feet wide so it will have 4 ft of deck all the way around it I'm going to run the stock motor tranny's 454 chevy / allison to a boat shaft and prop using a strut cutlass bearing with a s/s rudder should be a fun build if someone helps me with materials I'll let them use it for a week end in Mt desert island maine! (i don't mean one tank I mean half or all of materials for the pontoons)✌