I'll go out on the limb here.
I did my 51 bed last fall. Bought new sills, except for the rear one (the one under the tailgate). Each of the sills came with two flat square bracket/plates, that slide on to the sill, and has the slotted hole in them. These sit on top of your wooden block and rubber shim/spacer, and the mounting bolt goes through all of this. So to answer your question, yes, these are required. I suppose you could fabricate these, if yours are missing, but you WILL need them. Fabbing some would not be that hard, cause once it's all together and bolted in, they shouldn't move anyway.
Should be four sills total, counting the one under the tailgate, but this one under the tailgate is welded to the bedsides/stake pockets, and it does not use these plates, (it has the metal boxes made on it). The front sill bolts to the front bed panel, and then there should be two more sills kinda evenly spaced front to back (holes on the frame rails tell you where they go). Each of these sills use two plates each.