I just went through this (White Ind. actually provided one sump tank for me). Two options: One, ship it to Acorn Welding (Canada) and have them overhaul it (I had to do this for my 73G mid right tank). I think it was ~$2k total. They split it, fixed any leaks, etc. Second option, Aviation Enterprises manufactures tanks out of composite now for $3500 ea. Will never corrode/leak.
http://www.cessnaskymaster.com/fuel-tanks.html