For information regarding modifying your Fiero tachometer to work with any size engine, I recommend viewing this page created by Oliver Scholz.
Using the info from the website above, you can not only recalibrate your tach for any number of cylinders, but you can also create custom dials and recalibrate to extend the range. Here is an example of one that I did for a 3.4 DOHC Fiero with a 7000 rpm redline.
The original numbers and hashes were removed using lacquer thinner on a rag and careful rubbing, leaving a solid black background for the new numbers. I used Datak dry transfer numbers which simply rub on using a burnishing tool or pen. The dry transfers also come with various dial markings. The new numbers are "locked" in place using Datak matte coating that closely matches the OEM finish. Obviously this method will not work on the backlit gauges of the GT.