The width of the wear surface of a drum is a little wider than the shoes. this allows for some wiggle room so the edges of the shoes don't get hung up on the sides. Putting wider or narrower shoes on a given backing plate throws the geometry of everything out of whack. You need the shoes to sit square with the backing plate and drum. You also want the wheel cylinder push rods to push straight....not at an angle. The hardware has to be correct to provide the proper tension of the return springs on the shoes. The bell was added to the earlier A-body drum to add cooling capacity. The drums without the bell work just fine however.