[mr48chev]

Not a big fan of Poly tanks but most full size Dodge Vans in the late 80's use one. Too big for your use though.

Welding shops make fuel tanks for trucks all the time or at least they do around here. I've got three of them out here plus a fourth that was made to give an 89 ElCamino a 40 something gallon fuel capacity that I cut up. That was in a truck that had belonged to a guy who traveled between Arizona and Southern California on a regulars basis as the receipt for building the tank was still in the glove box and he had receipts from AZ and Cali in the glove box with it. I figured that guy was slipping over to Mexico to buy cheap gas at the time or at least picking stations with cheaper gas.
The other tanks are a dealer installed "extra" tank that was installed in 67/72 Chev pickups and two tanks that go in the front of the truck bed to increase the capacity.

AfterI pulled the stock tank out I'd snag a furniture box or fridge box (big cardboard) and measure and form up a cardboard tank that I could fit and try in the spot. Once you get it the size and shape you want that fits the way you want, mark where the filler goes and where you want the sender placed and take that to the metal shop. That way you know your measurements are right and have it figured out how you can mount it. You can have tabs welded on to mount it if needed.

The other option is to either search though listings for mini truck gas tanks online or crawl under mini trucks at pick a part to see what you can find that is close but a lot of those older smaller trucks had real small tanks. Not much fun on a road trip with a truck that gets 16 mpg that has a ten gallon tank. I've been there and done that.