The rail ended up being about a foot and a half off center.
Mount garage door opener off center.
My garage door opener came with the following instructions.
I mounted one off center for a door on my own garage that was 16 wide and it worked fine.
If your door has extension springs the opener must be installed in the center of the door.
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.
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.