I just finished installing a used chin spoiler on my '87 Targa, and I thought I'd share some experiences (in 10 easy? steps). I decided to install without removing the front valence from the car (next time probably would remove the valence). Below, I'll give, in outline form, my procedure.
1) raise the front end. I used drive-up ramps, which I recommend for their ease of use and their stability. Just remember to leave the car in 1st gear, apply the parking brake, and place a block of wood behind the rear wheels.
2) If your car has an air conditioning evaporator up front, then you'll have to remove the metal bar that protects it.
It is connected by two large bolts on either side, and by a small bolt accessible from the fron of the trunk (see pics).
3) Next, mark the positions of the where the embedded studs on each end of the chin spoiler will go through the ends of the valence. I made a template out of a thin piece of cardboard to aid in the process.
4) Drill holes with a 5/16 bit and install ends of chin spoiler securing with a washer + lock washer and a 6mm nut (10 mm wrench).
5) Support middle of spoiler with spring binder clamps, and prepare to drill holes. I used a 5/16 drill, and erred on the side of drilling the holes slightly toward the center of the car (i.e. towards the rear). The mounting brackets fit better this way.
6) Attach the mounting clamps. This is TOUGH, as there is very little room to get your fingers in to place the nuts. Also, tightening with the 10mm wrench is a b-i-itch. I was only able to get enough room to secure the outer four brackets (two on each side). The three in the center were essentially impossible to place given the cramped space. I did manage to get a middle clamp in place by punching out the stud from the clamp and using a carriage bolt in its place. That enabled me to place the bolt downward, and secure the nut from below. I didn't think it was necessary to use the final two brackets on either side of the middle. I did however use a couple of 1/4"X1" carriage bolts on each side near the ends for added support. I used a spray grease to lubricate the freshly drilled holes to offer some rust protection. Summary advice: If I had it to do all over again, I would have removed the metal cage that surrounds the evaporator, or I would have removed the valence.
7) Reattach the air condo evaporator's protective bar, and drive car off ramps....
8) Finished installation:
9) Double-check that all bolts are tight, but not too tight, as you may damage the rubber.
10) Consume the requisite 1-3 beers.
Epilogue: The job took me longer than I expected given that I was up and down looking for tools and hardware far more than I should have had to (I'm not terribly organized). Also, I like to work methodically, double and triple checking everything before proceding. Working on my back -- up and down -- for 4 hours is hard on me at my age. I must of done hundreds of "sit-ups" during the procedure
All-in-all, though, a fun time!