I wonder if a 16 gallon tank from a mid 80s through mid 90s short bed, single cab truck, would fit.

Seems like you should be getting better mileage though. You might look into tuning up the motor, rebuilding the carb, and making sure your tire size/gearing is as close to stock as possible.

Another alternative is an auxilery gas tank that fits in the spare tire location. Should give you between 10-13 extra gallons. The spare tire will still fit, but will hang slightly lower. Northwest Metal Products makes them. I think 4Crawler has one too. (whoops, looks like he beat me to it <img src="/forums/images/graemlins/smile.gif" alt="" />)