I have NOS (new old stock) 19 gallon steel gas tank kit available. It was originally made to mount UNDER a 60's-70's Chevy of Ford pickup, in the front corner of the pickup bed/box. I think the style is called a saddle bag tank.
Its rough measurements are 36x14x12. It comes with a new sending unit (for a Ford), a manual 4-way diverter valve (for THREE tanks), super long mounting bolts (thru 2 holes in the tank), switch and wiring for the sending unit (so both tanks can use one dash gauge), installation instructions, tank stickers, and a fuel filler neck. I think the neck is designed to go up through the truck bed floor. Since this is a NEW tank, you could also weld a new neck to the SIDE of the tank, for running a different filler neck and hoses.
I am asking $100 for this tank setup.
Here are pics of the tank itself ... I don't have pics of the accessories yet.