It does sound like you are affraid and tense when you are out with him because you are so affraid of what is going to happen instead of asserting yourself and thinking positively that you are in control of the dog.
The dog recognizes the fear and turns that energy into aggresion.
I too would try the umbilical for a couple of weeks around the house. I would try for an hour a day.
He needs to respect and trust you and you have to be confident.
Also, the time of the correction is the most important thing.
When walking if you trying and correct the lunging with such a large dog... or any dog it's really difficult.
But for example, if you know that before the lunch there is a pull, and before the pull there is a turn of the head, correct the turn of the head.
Keep his attention on you and him focused on the walk.
It will take time, but you steping up to be the Pack Leader/Alpha and be confident and not worried about all the things that may happen will definately happen

You can do it!
Keep us posted!
Rebecca