[Christ]

I also don't remember a Fiero with a V8 factory option...

That point aside, if you put wide fenders on, you'll significantly increase frontal area, so much so that covering the wheel openings may only get you back to where you started in terms of CdA. (drag per unit area)

The 88 GT, AFAIK, ALREADY has the lowest bumper/air dam available as an OE option.

The spoiler is necessary for the shape of the car-don't remove it unless you build a fast back.

Start with a belly pan from front to at least 2/3 back, tapering the last 1/3 up by about an inch or two if you do the rear portion.

If you want to test a fast back before wasting your money on a kit, just duct tape cardboard from the roof a and side glass edges to the spoiler, which is very close to 15°. Taper in the top section so the edges line up with the feet of the spoiler at it's rear edge. This gets you close to 10° side taper, also optimal.