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.
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.
My garage door opener came with the following instructions.
I had to do it that way because i had virtually no room over head and the spring was set right in the center not allowing me to center the rail.
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.
To offset mount the opener arm to the door section.