I would expect the frame on your '69 is pretty similar to the one on my '73. On mine, I had cross members ever 2' apart, and they would alternate between being flush with the top of the frame rails, and being ~3/4" below the frame rails. The ones that are 3/4' low are the places where two sheets come together. The reason they are low is so that you can put a ~4" wide board underneath the seam and screw the sheets to it to form the butt joint. The cross members that are level with the frame rails should run across your sheets at the midpoint. I did have one sheet in my trailer that was not 4' wide, it was only 2', so there were two recessed frame rails to match that 2' distance.