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They do make both panels, and you can either get it as one whole assembly, or as 2 seperate panels. The one piece assembly for a coupe/fastback is part #FTP65 from Mustangs Unlimited. I found it in their catalog, and you can see a picture of it at their website. It comes with full rear frame rails & torque boxes installed, and it has part of the front ones in place, and the front seat risers installed.
As for the rarity, the 200CID has become in the 66 Mustangs, rarer than the Hi-Po 289 due to the fact that just about everyone takes out the 6 cylinder to drop in an 8 cylinder. That coupled with the fact that it is a fastback, makes it unbelievably one of a kind, and extremely rare.