Is it possible you have the cables going to the wrong places on the carb assembly? One is a pull to rev up, the other goes on the other side, and pulls to rev down. I'm sure you've checked that already, but you never know.
The other alternative may be to loosen the jam nut on that side, scoot the jacket back as far as you can, and then see if it fits. On mine, I had to put the jacket in place, actuate the carbs to full throttle, then put the cable end in the fitting. It gets tricky putting the return cable in place, though.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.