I think there is some difference in manufacturing(obviously) between a post and a hardtop. I Have heard that the post coupes are "more rigid" and better to make into drag cars. I do believe there is reinforcement on a hardtop that may not be in a post coupe. Does a post coupe have the cross bracing behind the back seat? also does a hardtop have more structural enforcement in the roof or A pillars? I would think to convert a post to hardtop, this structural reinforcement would have to be mimicked. You couldn't just cut the posts out for example, I think that would lead to problems. I would think the best way is to remove the post top and graft a hardtop from another car, as well as all reinforcements/ braces. Its alot of work though