You would need to find most of the parts "used" since no one is reproducing much for the converts. Even wheelhouses are unavailable. Seems like I did see someone reproducing some of the windshield frame moldings. They usually go for $1000 or more for a nice used set. The quarters have alignment wedges in the door openings that align to pieces on the doors. All of the side glass is different as well, but it's available in repros. If you decide to go ahead with the project, I would suggest building a full sub-frame from mild steel tubing. I really think that it could be done if you have the patience and ability. You could build what I call a parade car that has no folding top. You could have an upholstery shop make a cover in case it starts raining or to just cover the interior up at night. I've seen several 57 chevies built this way.
If this 69 camaro rebody thing takes hold, I think that we may well eventually see cuda converts being reproduced. I think the demand is growing rapidly. I owned a 70 barracuda convert and a 70 chal convert and only kept the chal convert. Now I wish I had kept the barracuda convert as well. I would love to convert my 71 cuda to a convertible, but it scares me to think of ruining my car trying.
If you do it, we would love to follow your progress.