I have a 98 f-150 and am looking into buying a sub box...the one i found for cheap says for 2000-2003 extended cab f-150 but i thought all 97-03's were the same?? would this box fit without 2 much trouble do you think??

 

Ford changed the rear seat design in 2000. The 97-99s have the seats that fold forward and lay into a flat metal area. The 2000-20003s have the seat bottoms that flip up against the seat back.

You could run into problems with teh box hitting the seat risers, as well as physically fitting under the seat. There is a touch more room hieght wise under the 2000+ models compared to the 97-99s.
Dont take my word on that for sure, Im just comparing my old 99 to a friends 00 from what I can remember.