Take a look at what I did. I have great heat using the original heater ducts.
http://forums.pelicanparts.com/porsche-engine-conversion-tech-forum/556905-89-911-ls1-conversion-22.html
post 423 has a good photo but I didn't need the fan setup on the left. Just the silicon hose connections to the original flapper where the old heat exchangers connected. The interior fans worked fine pulling the hot air into the normal interior ducts. Good luck!