I think most of the companies that made "kits" for it are either gone, or won't do it anymore. It was VERY expensive, around $4k to do it right so that it doesn't leak, but there were all kinds of issues. Cars literally twisted so bad that the roof glass piece wouldn't fit anymore. And while adding subframes offered some relief, it didn't fully eliminate it.
Now, if you still want it done, find a custom shop, pay them about $4-$5k, and they will do it for you! For that money, you could probably find 96-04 GT convertible.....Found 7 (craigslist) within 100 miles of me between 1996 and 2004 GT convertibles for less than $5k.