Fiero seats are a bit on the small side so most everything else fills up the space from door to console a bit more. Also you may find that the bottom cushion on most is thicker (less headroom) and longer (hit your thigh at a different spot). Most small -> midsize GM will fit in and look good but you might want to try before you buy.
The standard seats are not hard to re-cover and there always seem to be lots of folks selling covers. The seat backs/ cushions can be swapped with a bit of effort and you'll have a nice set of stock seats.
I find the Fiero seats VERY comfortable and even after a long drive don't have a sore back as I usually do.
Gary