The original seat bolt pattern for the C3
is wider than the Fiero seat.
These plates bolt to the top
of the original tracks,,,
and have a second set of holes
that line up with the bottom
of a Fiero seat.
 

From Binnie at  http://corvettec3.ca/fiero.htm 
   Just a few explanatory notes about the brackets. The two top brackets are identical and the two bottom brackets are identical. Both sets of C3 tracks are identical so you need a pair for each seat. All brackets are drilled and tapped in the same configuration. Please note that two of the brackets have a 1/4" curved indentation. This is to allow clearance for the C3 seat adjust lever. Put that indentation in your brackets before you drill the holes. The smaller holes (for attaching C3 tracks to brackets) are drilled 17/64" and tapped 8 X 1.25 mm. The larger holes (for attaching seat to brackets) are drilled 21/64". The longer metric Allen head machine screws attach the brackets to the seats and the short ones attach the tracks to the brackets. The end with the holes directly opposite one another go to the rear of your seats. The centers of the larger holes for the seat bolts are drilled 1/2" from the edge while the centers of the smaller holes for the track bolts are drilled 3/8" from the other edge. These dimensions will center your seat tracks on the floor mounting holes of your car.

If you do not wish to fabricate the brackets yourself, I always have a kit ready to ship out which includes the four painted brackets and bolts - $115.00 and includes free shipping within Canada and USA. You can purchase a set of these brackets on my website at   http://corvettec3.ca/fiero.htm   with PayPal.