Although, it is your car and you can do whatever you want...
The general consensus at Team Chevelle is that if you want a convertible, then go buy one. Hacking a coupe to produce a convert will either cost you a fortune (to strengthen the frame) or will just wreck the car eventually (when the frame sags).
A coupe relies on its roof for structural rigidity. A convertible has a stronger frame to accomodate the lack of a hard roof.
There was an outfit converting the #@&% mustang from coupe to conv. Whether they performed this on others I don't know, may just be a #@&% thing. Would think it would cost big bucks. Or send it to Boyd he would do a nice job.
If I'm not mistaken, the Monte Carlo used the 116" wheelbase frame (like the El Camino and 4-door Chevelle). The Chevelle coupe used the 112" frame. So, if you wanted to build a Monte Carlo convertible, you could use a fully boxed El Camino frame.
PS: Chevrolet thought about producing a Monte Carlo convertible. They went so far as to assign a VIN prefix for that model.
I have a friend who bought a 69 Camaro that had undergone a convertible conversion. The car had a bunch of strengthening work done underneath, but eventually (after some hard use) it started sagging, pretty soon the doors were rubbing, then they started opening harder (sticking). He blamed it on the fact that he beat on the car too much, but I am convinced that even with the most gentle handling, a car like this is doomed to sag, yeah you can go easy on it when driving, but gravity and turning in and out of driveways (where one front corner of the car always goes in higher than the other) are ever present demons, looking to wreck your ride. I do realize that you would be better off with a full frame car like the Chevelle instead of the Camaro, but like I said, this car had a lot of good (expensive looking) fabrication done underneath, and still went to hell.
I get calls all the time from people wanting to do this to their cars. A convertible is engineered at the factory as a convertible. In addition to the obvious bracing under the car, there are dozens of brackets and braces in the body to help provide a degree of rigidity. The only way to do this correctly is to have a complete donor car so that EVERY necessary part can be installed. The cost of the labor to do this conversion usually exceeds the cost of a nice unmolested convertible.
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