I installed one... The fit is very good, and the overall quality is excellent. The toughest part of the job is dealing with the rain gutters. The roof skin is pinchwelded the whole length of the gutter, so grinding the top layer off the gutter channel with out disturbing the lower part of the gutter is one of the tough parts. I used a thin ( ~0.060" thick) cut off blade on my 4-1/2" grinder. Just lightly grind away the pinch weld and it goes ok. Welding the roof back on is easy at the windsheild, rear window and along the quarter... once again the rain gutter is a little tricky. I used a combination of spot welds and mig welds to get a solid continuous weld.
Yes, It's exactly what I did... My coupe roof was severly shot, so I got the inner roof structure from a worn out fastback and grafted it on to the coupes platform. With the roof skin off it made it a lot easier to properly tie the upper roof rails into the A pillars at the factory joints. At least two others here on VMF have done coupe to fastback conversions.
Key point here is that the conversions have all used existing fastback inner structures. While all of the fastback outer sheetmetal is available, the inner structure of the B-pillar and rear inner quarters has not been released yet, and I don't see it being released any time soon. You will need this inner structure to attach the upper vent/sail panel, and to attach all of the interior fiberglass panels.
If your plan is to do a coupe to fastback conversion strictly using reproduction parts, you are not going to be successful unless you can do one heck of a fabrication job on the innner structure.
I believe Dynacorn has specifically stated some of those parts will not be released, ie they are keeping them as proprietary to protect their complete bodies (doesn't help their business model if they sell you everything you need to build your own unibody cheaper than you can buy theirs).