I have a 5 speed out of an 07 Charger in my GTX behind a 5.7 hemi. What a blast to drive and the mileage is great. MAJOR pain in the ass to get it to work without the stock speed sensors. Finally found a guy who wrote a program to manage this type of swap and that is what made it work. Nothing else we tried got us out of limp mode.
You might want to talk to him. I don't know how much experience, if any, he has with the 8 speeds but it is worth asking.
Russell Drake
www.soundgermanautomotive.com