My garage door opener came with the following instructions.
Mount garage door opener off center.
If your door has extension springs the opener must be installed in the center of the door.
I mounted one off center for a door on my own garage that was 16 wide and it worked fine.
The rail ended up being about a foot and a half off center.
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.
As you said run perforated angle iron from the center stile to the next stile over.
Use one piece as near to the top as you can which would be just below the strut.
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.