I only know of one person that made a convertible out of a 3rd gen f-car and he was adament he'd never do it again. The car is so heavy so ALOT of bracing is required, which of course makes it even heavier. Of course, it still wasn't enough. Think about the cowl shake you've got WITH the roof on.
Another option, a bit safer, is to build a (modified) Pro-stock/sportsman chassis using original suspension and then attach the body. that way you've got a strong frame in case of an accident and the weakness of the body is irrelevant.
Just my .02, no matter what, good luck!