You'd need to cut the roof and B/C pillars out. Strengthen the frame, factory convertibles had front and rear torqueboxes (will further strengthen the frame in my 70 rr droptop don't want any flex). Your wheel well houses will need to be modified to fit the convertible, (can get more info on that later, modifying a set from AMD to replace my rotted ones). Modify the rail above your windshield to allow for a top to latch down. Find a top (good luck finding one...). Get, route, and wire in the hydraulics. Make some mounts to for the top. Shorten your back seat width wise so the top can fold down. Cut the package tray out. Weld in a support for the top. modify your Dutchmen panel to allow the top to rise up.
There's likely a lot more, have hardly started doing my convertible, may be able to provide you with more info as time goes on. Will be more than happy to get you pictures if you need them.