Ah memories.....Did you ever get the ZX8 working, or get the 7 motor in the 6 frame? Did your friend ever get his shop?
The frames are different based on their air induction and the the ports in the front of the frame more power more ports through to the front farren section and also differences in the topend an radiator size. When you go in compression ratio I believe it also ups you coolant also on the 7 your motor mounts are you creshpegs as well or rollers. The 9 into the 7 is easy they make a kit for it....just do that trust me you won't be that fast. You'll be heavy as hell and you'll get passed up around the twists and turns by the 600/750 some dude catching a good hot tire with some decent rpm is going to be knee down going passed you like a missile he's Gunna look back at you for a split second and that's when you woulda wished you would have built the 7 in 7 frame instead of being one of those people that hacks up super bike gold and still doesn't end up with a working bikethe reason it would fit into the 9 and not the 6 would be due to the difference in the size of the frame. Why ask a question then not listen when someone tries to tell you something?
I think someone did try to do it or DID do it and it required shaving some of the frame and cutting and relocating the upper motor mounts, or maybe im thinking of a diff bike/motor combo but I was pretty sure thats what it was.
and what does the tail on the 650 have to do with your original questioN?