I have a 91 F250 S/C long box w/dual tanks. the rear tank leaks when over 1/2 full.
Will a tank from a 90 F150 SB work? are they close enough other than capacity?

 

Pretty sure all the rear tanks are the same, its the fronts that are different,and even those can interchange.

 

The rear tanks can be either plastic or steel - Either can fit (with the correct straps) but the FDM is different for the two, so you wouldn't be able to take the fuel pump/gauge sender out of a plastic tank and put it in a steel tank

Don

 

Check the replacement tank closely for rust if they use salt in your part of Canada. Your leak is probably due to rust, most likely under a strap or at a seam.