how thick is my sheet?

I would like to hang a propane tank under my all aluminum step van that has been converted into a food truck. The tank will hold 37 gallons and is 42 inches long and 16 inches across. At 80% propane fill and including the tare weight of the tank we are looking at 200 pounds. I am thinking it would be best to distribute the weight of the tank across multiple beams (also aluminum) that support the floor of the truck. If I were to use a sheet of aluminum 33 inches long and 16 inches across, how thick would it need to be to support the weight of this tank (that tank has four attachment points along the top. Any help would be appreciated.