Larger fuel tank
 

I have a 1999 F250 SD Crew Cab Short box. V-10. I read somewhere here that the larger fuel tanks from a long box will fit. Anyone tried this or know where to direct me to look? Really appreciate any help before I go buy one and find out.

They wont fit. Not without major truck modifications.. But an Excursion tank will fit much better, and carry more fuel (40 gal) however you will sacrifice your spare tire. I suspect you dont want to spend $1000 on an aftermarket one. Not sure how well they would fit but maybe some 450+ tanks will fit, I think they have different tanks, not sure though.

I did see on a site there was a tank that fits where the spare tire goes. How does it tie into the fuel system?

I hope you are not confusing tanks used on flatbeds with the others. I do have 35 gallons tank behind my rear axle, but my bed frame is 10 feet.
Big tanks are not totally convenient. With fuel down to 10% you have to watch the tilt, or you will suck air with 5 gallons of fuel. Consider dual tank option.