Here's what I did several years ago.
http://www.jrmock.com/fuel_cell_install_1.htm
Inspected cell interior late last year, and everything still looks good inside. The 8 Gal. works for sprint races, I've run several 45 minute races, and been fine.
If you go with anything bigger, you are looking at a custom fuel cell, or different placement. (I've seen one car with a 15 gal cell above the transaxle, with custom bulkhead to keep it seperated from car interior, and another car had one mounted in passenger area, again custom bulkhead and fueling arrangements.)
good luck,
JM
