Howard,
I think you are doing it backwards.
Unbolt the end that is already fastened. Determine where level is and mark it on your post. Then attach that end first. It should look exactly oposite of the picture you posted.
The overhang will be way in the air and the other end will be on the dock. then it will be a simple matter of lifting that end up and fastening it to the post.
If it was me, I'd remove your existing beams and install the overhang one first. Then all you have to do it make sure it's level with each other. Cut the beam so it meets halfway on the post and install it. You can use brackets to make it stronger if that's a concern, but having your beams all level will give you a much cleaner look. Then it should be pretty simple to put the already installed beams back up.
How far are those 2x4's apart? I've found that at 8 feet, they will sag over time. You might want to reconsider them and put use 2x6's instead. I'd also space them two feet apart at the minimum if you plan to put bamboo over them. The bamboo will sag really fast and two feet still might be too far apart.
Eddie
