i have a 90 lumina im going to do this to, & i figure being the same platform that the roofs should be the same-im yet to measure this but if they are different i highly doubt it will be much of a changover (id be using the lumina for the mold/molds & then modifying them for the gps as well as cutlasses/sunbirds if people are interested.
also in regards to the comments on the worries about messing up your car, i would include a detailed instruction video & pictures on how to hook it up, as well as a piece of rubber/similar trim to go on top of the cut metal to make everything look nice even if your cuts do not. the big thing is, the worst thing you could do if you mess up & cut a little deep is youd need to fiberglass/bondo the messup-sand it-& touch it up, the skill level involved here wouldnt need to be very high, you will just need to take your time on your cuts so that they are good enough to where the seal doesnt look really wobbly, & you could always throw down the strip-find high spots & grind em down to where they look nice, or fluff low spots out.
basically, if you cant paint, cut a semi-straight line, measure, drill, or use handtools of any sort-you probably shouldnt be making your car into a convertible-yourself. this would be for those out there thatd be willing to spend the time & effort to have a very nice/rare convertible (i mean seriously, how many convertible gps or luminas do you see every year? i dont know about you but any i might have seen have been poorly done hackjobs with sprayfoam & duct tape for seals-and thats just wrong (but funny!).
if i can come up with a working model & instructional dvd is anyone interested?