Cool idea! If you check the profiles of the coupe and the speedster you'll see that you'll be MUCH better off picking up a rear speedster clip and glassing it on. BUT, if you really want to try it in all metal, you can get most of the way there by converting the coupe into a cabriolet. The cab profile more closely matches the coupe and the passenger compartment opening is longer and less "turtle backed" so that you can cut and lower the coupe to match it a bit easier. The coupe windshield stays in place with a new top cap, and the side windows are also used "as is"
The benefit will be that even with the raised rear, you can just lower the back of the top bows and immitate a mild "chopped top" look to great effect.
Rather than use all Porsche sheet metal, you'd be well off grabbing up a Karmann Ghia Cabrio and using the inner bulkheads, top and frame, and probably windshield header as well and modify to fit. Save a LOT of money and end up with a nice look.
The coupester that Dale mentioned was a very awkward looking race only car and I think that the folks at Parts Obsolete were owners or builders or involved in some way at one point. If they were, it must have gone like stink, even if it looked pretty stinky doing so.