The bolt that holds it on is TTY, which means you cannot re-use it. Sometimes there is a procedure to the bolt tightening as well (not sure about your 98 though) for example:
14520
4.6L 4V Pulley Instructions
Some 1996-2001 Mustang owners, for engine performance purposes, are adding a special front drive pulley kit to reduce front accessory drive speeds. When removing and replacing the crank pulley, if the large bolt used to hold the crank pulley undertorqued, the keyway will shear and cause major engine damage. The resulting engine damage is not warrantable. If the crankshaft pulley bolt is removed it should be replaced as it is a torque to yield bolt. The installation of the crankshaft pulley requires that a specific torque operation be adhered to. The crankshaft pulley bolt must be installed using these 4 steps: 1) Torque to 75 lbs/ft 2) Back out one turn 3) torque to 45 lb?ft 4) continue to tighten and additional 90 degrees.
when i put on my underdrive pulleys i torqued it to 80 lbs then back it of about 40 lbs then torgued it a good deal with a bit of impact wrench action to seal it tight given the crank was turning then and i didnt want to mess w/ the solinoid