Yeah I agree that looks simple and straight-forward enough if You know Your bodywork, I was trying to suggest You´d really do well by using the frame of that convertible to set it all on top of, as well as study the ´vert underpinnings for what the unseen differences between the open- and hardtop structures are. Convertibles of this era always had more body-to-frame -mounts, thicker gauge frame rails, extra reinforcement -plates on the bottom of the mid-"X"´s in the case of the GM´s of this vintage etc.etc. (designed to make up for the lack of roof keeping the body from flexing and twisting all over the place going through turns, over potholes and such.) In order to come up with something as close to a factory-made ´vert with all that work, that´s where I´d start anyway. Frame. That should also work as Your "jig" for properly lining up all the various sections You´ll be joining from the two separate bodies. Best of luck, post updates as You go.