I have an awning on my 78 surfside
trailer, unfortunately I am at work right now and cant measure the rail height. I will post another message tomorrow with an answer. The main criteria for the awning would be to install it so that it slopes away from the trailer and also allows headroom clearance at the outside edge. The awning also has to be high enough that the top of your trailer door doesn't rub against the awning during opening and closing of your trailer door. If your awing has walls, you will have to set the awning height according to the height of your wall panels.
I think that I would prefer large head rivets to eliminate any chance of the rail coming loose due to vibration. The fiberglass isn't thick enough to allow a screw of any length without something added to the inside of the trailer to screw into. If you use nuts and bolts, I would suggest stainless steel hardware and use the nuts that have the nylon inserts that prevent the nut from loosening on its own.
I hope this helps