You cant go by NADA values, as they are pretty much bogus. If you check places like autabuy.com, you will find dozens of beautifully restored 1970 340 Cudas for $30k-$40k. You might get that out of a survivor, though, so your price is not out of line if the car is as nice as you say.
As for making it into a convertible, you would have to put so much money in it that it wouldnt be worth it, and if you ever went to sell it, no one that knows anything about them would want to buy it. Totally a lose/lose situation if you go that route.
Since you say you can only afford one personal muscle car, it seems to me that your best course of action is to sell the Cuda, and the Chevelle. That should get you into the neighborhood of a Cuda convertible that needs some restoration or TLC, and you can do the rest. Dream realized. Problem solved.
BTW, Challenger convertibles go for a lot less than Cudas, so you could also just get a Challenger. It's still an ebody, and not that much different. One of the board members recently sold one that needed restoration for a couple thousand dollars.