I did the inside skin of my trailer pretty much in the manner you describe, I have images and descriptions on this thread:
I didn't need to make extra ribs for the curved ends in my 62' trailer since I worked over the top of the fiberglass endcaps with my birch plywood. For the main body, I removed the aluminum, reinsulated, rewired, resealed, and then replaced with the plywood. I then made new trim for the windows. It was a long process but I like the results. I would worry a bit about weight with both birch and aluminum layers unless you use 1/16" ply when you go over the interior aluminum (which might be a good idea - easy to form and install). I have the patterns for the 13 panel endcaps if you want paper copies - they might need some tweaking to fit your trailer (or might not work at all if the curve is different enough) but they worked in both the rear and front of mine and those areas are fairly different. As Tinsel Loaf says, if you make paper patterns out of tarpaper or red rosin paper you can tape them into place to fit before cutting your aluminum or birch.