For what you will spend on the conversion, start to finish, to have it done right, you could restore the hard top car and call it a day. Or, you could build a damn nice engine/trans combo to drop in. But it would look good, and you would have the drop top that you want. It's all in how much you're willing o spend.
Another option would be to sell the car you have and locate a convertable. (they are out there, I had a '68, my uncle has a '70)How much would it cost to change my Chevelle into a convertible?Sure it can be done, a trim shop could do this but finding one that's willing to spend the time and know- how to do this is another question.
First you have to find a frame and from past experience dealing with older cars parts are not cheap and not so easy to find. So you'll be paying dearly.
Just to give you an idea my family owns a custom trip shop in the midwest if this was a convertible to begin with the top would run 1100.00 - 1600.00 depends on type of material you want, not including the back glass, or regal light( plastic back window) add another 300 for plastic or 600.00 for glass.
So your talking about cutting roof off, building a dam (top hides into when down), getting a frame, installing a top along with the hydraulics for the top.
6k-8k it all depends on parts (frame)
To be honest you'll lower the value of your car being it's not a true convertible.How much would it cost to change my Chevelle into a convertible?Possible? Yes.
I personally wouldn't attempt it unless I had a salvaged donor vehicle of the same make and model, but a convertible.
Without a salvaged donor car, I suspect that you would be spending far more time hunting down parts and components then actually working on the conversion.How much would it cost to change my Chevelle into a convertible?Why would you want to destroy the value of a classic by chopping off the roof? Why not just buy a convertible in the first place? The cost of doing it and the decrease in value of the car from the modification would make it a very bad choice.How much would it cost to change my Chevelle into a convertible?It can be done but be practical the cost would be astronomic.