'79 Newell Water Tank Leaking - You have a number of alternatives. A metal fab shop in your area can probably duplicate the stainless tank. Probably an expensive solution. You might also have a custom tank made out of heavy sheet plastic. We just did a web search and found this supplier:
http://www.all-rite.com/customtanks.html You might also do a search to see if you can find a vendor who has a moulded plastic tank in a size that you can use. From time to time, we have seen ads for vendors offering a variety of tanks in magazines like Bus Conversion and FMCA.