I'm not sure of the year of the Fiero, one model had a longer hub. There seems to be plenty of them in the wrecking yards. The spline on the shaft is the same so it is a bolt on. The center cap is from any 70's Camaro ralley wheel also the same on Chevettes. I haven't figured how to post to this site yet but if you send me a email I can send you a picture.
Fred

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine