I'll save you and Soos some time since I've read most of his posts and swapped the carbs myself. I put some early 70's 750 carbs on my 80 650 and ran into some problems. The cable holders on the 750 are too far forward you'll have to do something about that, air intake side is about 5mm smaller in diameter on the 650 so the 650 boots either have to be clamped rediculously tight (i worried about them not sealing properly, and long term about cracking in the rubber) or you'll have to either modify the intake diameters (I toyed with the idea of having pvc machined down to do the job) or switch to Pods (which is ultimately what I did, I can not comment on whether they are better or worse than the airbox as I really haven't done more than idle with them.) The other problem with the 750 carbs is they are shorter front to back, I found some marine grade fuel/exhaust hose that I'd made some boots out of, but that presents some other issues as well. If you choose to go this route and have any questions feel free to PM me or check out my build thread. I won't comment on whether this is the route to go or not, it was an individual choice because I hated the stupid choke cable and how expensive the 650 carbs are to rebuild, again, personal opinion.
Also from what I've read the spacing on the 650 cv carbs is different, I can't remember if there were any diameter issues or not, and I think they are taller. I just know that from what I read it is far from a direct drop in replacement, otherwise Soos would likely have ran them. He is currently running Ninja 600 carbs on his hopped up 650.
Can you not fab up some sort of cable holder for the 650 carbs? Sorry, I gave both my sets to Soos or else I'd see if I could come up with any helpful suggestions.
Brandon