I'm not 100% on this but I believe you need to purchase a 987 or 996 plenum thats why you have to use the 75mm throttle body which is stock on a cayman and 996 - you need to find someone who has done it allready, I have a TB waiting but have not had the time to spend $900 for a possible 5 hp. what you can do is go to Pedros garage and see what he has to offer, I just bought one from him and am about to install it, let you know how it works. I have talked with at least 2 people you have installed this and said it worked very well
http://pedrosgarage.com/Site_2/TechnoTorque.html