PLEASE use a case that is not an 'S' VIN. You'll thank yourself later. Uncut cases on matching 'S' cars are already bringing hefty premiums and a 'T' or 7R case will serve you just as well. Spending a few hundred bucks on a less valuable case now will put thousands in your pocket later should you decide to sell the car.
(edit) I just read your post more thoroughly. Geez, I hate to sound like an 'S' snob, but you need to reconsider this project. Hacking up an 'S' like this is like burning dollar bills. Every cut and body panel replacement you make on that car will decrease its value by the amount it would take to put it back to stock. Would you do these kinds of mods to a Ferrari Dino? I'd bet you could sell your 'S' at a tidy enough profit to build the race car of your dreams out of a less desirable car.
Go to the Early 911S Registry BBS and website for more info:
http://www.early911sregistry.org/index.html
http://d240157.u39.zeonhost.com/forum/index.php?