does anyone know if 55 plates will work on a 57 small bearing 9 inch rear end? I know that the bolt patten is the same/ its the center hole in plate that goes over the bearing that Im not sure about Any help on this would be greatly appreciate

The '56 uses a 62mm OD bearing, the '57 uses a 72mm, so obviously the hole in the backing plate is too small.  I don't see why you couldn't open the hole up enough to allow the bearing to go through it.  The other thing to consider is the length of the axles from the bearing to the brake drum flange.  If they are different, the alignment of the brake shoe and the drum may be wrong.

John - "The Hoosier Hurricane"