You cannot reuse the stock arm, it will be 5.5" (in your case) too short. You can use your current rear brake cable, you will need to spin it 180 degrees on the master cylinder behind the right passenger peg to allow it to have enough slack in it. This will make sense when you get the bike apart and try to visualize how the heck you're gonna get the cable to reach. There's enough there to get to about 6.5" over.
Some extension kits come with a bracket that allows you to not even have a brake bar. Others come with a longer bar, and some people have even made their own. Depends on what you got.
You'll need a longer chain. This was the hardest thing to do. Cut the old chain off, connect one end of the new one to the end of the old one, pull it through and around the front sprocket. Then button everything up. If you cut the old one off and pull it off without using it to thread the new one, you'll have to pull your sprocket cover off to wind the new chain through.
Like you said, it's straight forward, once you've seen how everything is put together.
Good luck.
I don't have any good pics during the install, but can show you that there is more than enough brake line there and my style didn't use a brake bar.