I have a 1998 f150 supercab/shortbed /4.6/4x4/16x7's
I have 255/70/16's on it now, how big of a tire can I get without it rubbing ?

 

You can easily clear 285/75-16's (33x11.50) using you stock wheels. If you plan on keeping the OEM wheels forever, this is probably your best choice. You can fit a larger tire and wheel combo by cranking up the torsion bars and trimming a little plastic off the front bumper's lower plasic valance.