I don't know if you are going for less weight with the 12 gallon or whatnot, but you could go balls-out and get the Cobra R 20 gallon tank.

I have a picture somewhere from Mustang Alley 2008 which is of a fuel cell in the trunk of a 98, I'll look for it and post it if I find it.

 

Where can you get a new Cobra R tank? and will it mount in the stock tanks place?

 

www.fuelsafe.com/pdf/mustang.pdf

 

A friend of mine has a 95 R. we recently replaced the fuel cell with a conventional tank.

Although the car only has 14,000 miles the rubber bladder was leaking. A replacment bladder from fuelsafe was over $1000, and a complete fuel cell is ocer $2000.

It mounts in the stock location , but the mounting straps are modified to fit the cell.