I like the concept of your diorama, but it will be difficult to pull it off successfully. As others have pointed out, the area of your diorama only covers 70 x 105 feet of beachhead in 1:35 scale.
Have you ever visited Omaha Beach? I was there last year and I was surprised at the width of the beach at low tide (remember that the landings began at low tide) and how exposed the area is; it was immediately obvious why the troops were pinned down progress across (and off) the beach was so devastatingly slow. Examine some photos and you will appreciate what I mean. Google Earth is a very good reference source.
Not only is the beach wide and flat, but the slope from the beach up to the crest of the hill where the bunkers were located is long, exposed and shallow.
I doubt you are really going to be able to show much of a realistic story with such a small area of the attack. Good luck with it, nevertheless.