Breaking stuff to look tough.

If you are referring to the tire to bottom of spring perch on the strut in the back.  1/2 inch is more than enough clearance.  My larger 16" tires, had less than an 1/8 inch between the strut tower and the tire.  Your wheel is bolted solid to the hub, the hub is bolted solid to the strut.  The distance between the strut and the tire is static and does not change.   The distance between the tire and the body does change, however in my life of Fiero's I have never heard of anyone being able to bottom a rear tire out into the frame. Even running 20" wheels.  

        Now to actually answer your question, Yes you can get a "coil over kit"  Which changes the spring perch diameter and spring to about 1/2 of the current size.   Its usually install for people that A. want to go racing and have the adjust ability of a coil over, or B. Someone that wants to put really frigging wide tires on the back, or C. Someone doing a Chevy V8 with steel heads and wants to keep the rear suspension a bit firmer. 

John Carlo I believe has coilovers on his car, as well as John Craker because he did an 88 Cradle Swap, and Lisa has them on her V8 Fiero. Hugh has them on his car for the tune-ability of them as well.

      Plan on spending at least $300 to do it right, or $150 if you just want to get the cheap sleeve kit and install it on your existing struts but cutting off the current spring perch.     I can fit a 245/50/16 on the back of my car without rubbing, which is more than enough tire for the back of a Fiero for performance purposes.