If your door has extension springs the opener must be installed in the center of the door.
Mount garage door opener off center.
The great thing with wall mount garage door openers like the chamberlain rj020 is that they save a lot of space when it comes to ceiling use a common problem with most ceiling mounted garage.
I mounted one off center for a door on my own garage that was 16 wide and it worked fine.
Use one piece as near to the top as you can which would be just below the strut.
As you said run perforated angle iron from the center stile to the next stile over.
The opener can be installed within 4 feet to the left or right of the door center if there is a torsion spring or center bearing plate in the way of the header bracket or door bracket area.
Make sure there is a strut on the top section it s required when using an opener run a second piece 6 to 12 below it.
My garage door opener came with the following instructions.