Build a fresh box section, I added couple formed pcs top and bottom of the box to strengthen. All 16ga HRS, overlapped, all seams welded, then secured to frame with formed angle sections, all overlapped and welded all seams, 3 layers gives you 3/16". Weld to floor also, replace floor if required. Paint all with a rust paint (you can't future weld thru spray rubber or ashpalt based coatings). Still workin great for me 1 year later. Lots of pics in my album section. Remember to remove tank and all flamable materials/fuels. Have a buddy of fire watch while welding.
Got any pics of your current condition?