Have a look at the following thread:
It is about the installation of an A&E awning on my '73 GT. I did some modifications to the awning to make it work, specifically, shortening the arms so that the lower mounts hit the side higher up, rather than at the belt line. The original design was for the lower mounts to attach right at the belt line using long lag bolts to go through the skin and the C or U cannel at the bottom of the wall. My problem was that on the rear mount, the mount landed right in the middle of the battery compartment door, so needed to be raised up. The upper mounts will straddle or sit near the ends of the awning rail, depending on the length of the awning rail relative to the length of the awning tube. It is a bit of an iterative process to get the placement just right. If you need more detailed pics, let me know what you want to see.