Let me start off by saying the F40 has a gated shifter built into the shift mechanism. There is a pin that rides in the gates in the center tube. This is one of the reasons for the notchy/clunky feel of shifting the F40.
A functional gated shifter plate has a few issues...
1. With new cables and rebuilt shifters there is still play in the side to side/front to back motion of the shifter shaft/knob. In the general raised area that this proposed gate is, the slop even with all new parts is still at least 1/8" or more. This means to avoid the shifter shaft from rubbing on the sides of the gates, there should be 1/8" of clearance on each side of the shifter shaft for the gates. Making the gates too wide, limits the material thickness separating the gates, which becomes a functional and aesthetic issue. Narrowing down the diameter of the shaft and raising the plate location helps balance things out, but there are practical limits.
2. Is when things wear, there will be more slop and the shifter shaft will start making contact with the sides of the gate (or the gate is designed for the shaft to rub the sides of the gate in the first place). The issue here is the contact will want to move the gate plate around as the shifter shaft bumps into it from all directions. The shift gate is attached to the chassis via the largest, most fragile, and difficult to replace piece of plastic in the Fiero (console skeleton) and the risk of breaking it is greatly increased with a functional gated shifter plate.