Northwest Metal Products sells a metal tank. Very $$$$. I think this is the tank Downey sells. I run NWMP tanks on both my mini's and really like them. I plan on their tank for my FJ40 project.
I am running the NWMP aux tank as my main tank on my '76 40. It is the same tank that Downey sells and the whole kit runs around $450, about $100 cheaper than the Downey kit because the skidplate is included. The tank is pretty heavy and seems well built. Skidplate itself is almost as heavy as the whole tank.
I ditched the stock tank altogether and I think the NWMP is about 24 gallons or so. Really nice not having that stinking, leaking thing under the passenger seat anymore.
The tank mounts underneath the bed, behind the rear axle and against the rear diagonal crossmembers. There are straps or a skidplate that sucks the whole thing up against the bottom of the bed and those mount to the shock crossmember and the rear bumper. The kit I bought came with a fillneck that uses a 2 1/2" hole in the rear fender and a large grommet to mount. I didn't want the fuel cap sticking out past the fender so I used a Confer fillneck that has a large square recess for the cap so it doesn't stick out to grab things.
I have a custom AUX tank with integrated skid plate, autometer fuel sending unit and guage, and electronic selector valve for sale. It is a 22 gallon unit. It also has a Con-Fer filler neck. Asking $250 plus shipping. As soon as I get my digi cam batteries in I can snap a pic. I know this should go in the for sale section, but since someone is in the market, I figured I'd offer up.