MK1 Pick Up Truck upgrade the gas tank question.

[climbinghalfdome]

So after 363,000 miles the gas tank is leaking. I've not spent a lot of time under that part of the truck, or looking from where is it leaking. What a wonderful excuse to switch out tanks and explore upgrade ideas. I've seen NOS replacement tanks on ebay and other places, however I'd like to see if I can get a bigger tank under there (to increase the volume I can transport) with as little modification as possible. Even better yet I'd like two separate tanks under the bed. One for regular diesel and one for WVO.
Do ya'll know of anyone who has boosted their fuel capacity? What tanks fit? Where'd they get them?
Kevin in the Grand Canyon

[honda_is_the_best]

god, i remember a guy that had a caddy with like a 24? gallon tank under the bed in the stock location. but i didnt care enough to pay attention to the particulars, like what it came out of or where it came from. but it can be done.