Hi Donsz,
I'll answer you from the perspective of my 55 second series. Hope it is the same as yours.
The bracket mounts to the bed cross sill just as you have said. The flat side of the bracket on my truck is forward with the channeled side toward the rear. This gives you information to distinguish right from left bracket. The holes on the cross sill are on the front with bolts going from the front through the cross sill then through the bracket and into captive nuts welded on the rear of the brackets.
If anything is not clear just ask. I will keep an eye out for you.
