I have an angle-iron installation with ceiling 4 foot higher than door like you describe. It works great. The angle-iron doesn't give like wood, so I would consider building the entire support structure out of angle-iron.
I have an angle-iron cube structure attached in all 4 corners to 2 studs. With the standard type of door opener, over half of the load is excerted at the front of the bar that is mounted to the wall above the garage door. Make sure this point is secure. As you state about tension exerted by the opener, this is the point that should absorb most of that tension.