I've done a few of those....the best way is to cut the arm at the same distance on both, and weld the double ball on the end of the original arm. It's important to ensure that the arm is centered fore and aft between the two carbs to ensure the opening rate is the same on each carb.
If you don't have access to welding ( I use a torch )then you can cut the single ball arm off right below the ball, and the double ball one at the kink. Then you have enough to overlap the two arms, and drill two holes for nut and bolt assembling. Use red loctite if you do this.
The choke cables clip on to the bodies like the later HS6, and the wire passes through the retained clamps on the fast idle arms.
Nice upgrade!
Rhys