I don't like the idea of the tank under my back side either but this does make you think...
http://339297.mobilizetoday.com/blog/index.php/category/technical-articles/1947-55/gastanks-1947-55/The 47/48 used a strap system that held the tank in place on the passenger side frame rail which is the same as 49 and up. Reproductions are available.
http://www.oldchevytrucks.com/cart/product.asp?prodid=MEG159&i=19888|||||
This tank was exposed to easy rock and road damage. If a tank was built from 16 gauge metal then it may not be a problem.
