I am piecing together a mikuni kit and but have a few questions.

I purchased brake cables for the throttle cable from wal-mart. It seems like its too long. Do I have to cut these? How have the rest of you made them work? If i cut an end off, how do I attach it to the lever afterwards?

See pic. What does this outlet go to? Its leaking gas. Do I need to plug it or does it go to a hose?

That is your over flow tube. Add a hose, and make sure it wont drip on your motor.
Mines leaks only when trimmed all the way down.
Also, if it is steadily leaking, you will have to adjust your float, letting in too much fuel.

And you can just cut your cable and make it work. Just make sure you are getting full travel on the carb too.