Coupe and conv bumper covers are interchangeable. So are the bumpers for that matter, but there are some differences. The convertible bumper does have weights in the outer ends. They weigh about 8 pounds each. The big difference is in the way they mount to the car. The coupe bumper mounts directly to the frame rail extensions at the front of the car. It mounts flusher(if that is a word) to the nose of the car, and uses plastic grill pieces that protrude evenly top and bottom. The convertible bumper mounts on 1 inch rubber mounts, and has a center pivot arm. It uses grill pieces that extend out farther on the bottom than on the top to fill in the gap created by the 1 inch spacers. The rubber mounts are sandwiched between two steel mounting plates. One set of bolts mount to the bumper and one set mounts to the frame rails. These delaminate over time. That is why you see converts with the bumpers sagging at the ends. I've heard the new replacement ones are junk.
For a brief time there were converts and coupes being made at the same time. The coupes made during that time used 1 inch aluminum spacers instead of the rubber mounts. Those late coupes also used the convert grills. You will also note a big difference on the frame rails themselves. The coupes just had a flimsy piece of steel connecting the two frame rails. The converts had a boxed member that was very strong. It was designed to carry the weight of the center pivot and also to stiffen the chassis. Since the coupe bumpers are a direct bolt on, they act as a frame stiffener. The convert's bumper just floats and adds nothing to the rigidity of the car.
Bolting on a coupe bumper and it's accompaning grills is the way to go. It lowers the overall weight of the car by about 20 pounds. It brings the center of mass closer to the center of the car. It decreases the weight suspended past the centerline of the front wheels, which helps with car's tendency to nose dive under heavy braking. It acts as an additional frame stiffener. And most of all it improves the apperance of the car by downplaying that ridiculous bumper sticking out so far. If you are real adventurous you can drill holes in the steel to lighten it, you can make an aluminum substructure instead of the steal one, or you can go full fiberglass. It's amazing what losing 60 pounds off thwe nose of a car will do for it's performance. My favorite method for a street driven car is to take the aluminum substructure from a SD1, pie cut and reweld to get the proper angle. Weld in the previously mentioned aluminum spacers, and remount the lightened rubber cover.
I've seen some pretty nasty covers brought back by light wet sanding with WD40 or some other chemical that breaks down the rubber. My favorite is that purple grease cleaner followed with a protectant.
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