I seriously toyed with the idea of a tank behind the read btwn the frame rails. Go to tanks.com and look at the 3 options for suburnad/chevy van tanks. Will fit perfectly between rail and you have options up to 31 gallons.
Positives: weight on rear end for traction, run for ever on 31 gallons, frees space behind seat. Look at Holvarths site - he has a nice install and pic of one.
Negatives, where did my spare go? I opted for under seat 17 gallon because it was factory, and primarily I could not find a good spot for my spare. I tried to fit spare back there could not do it so I left it alone. NO SPARE on FENDER- no discussion there.
They do fit in nice and clean on the installs I have seen.
Send me private note and I will send you a nice clean shot of an install. I will stick the picture on my Webshot link to. It is a easy swap and as so is much cheaper than any kind of kit. Heck you get a brannd new tank for 129 dollars... no rust and run.
Cavebull

cavebull
Stovebolts - so easy even a Cavebull can do it.
https://www.flickr.com/photos/dodsonbrett/collections