GM started welding the rear sill to the bed sides in '51, to eliminate the possibility of the sides spreading or bowing when loaded. I've seen either two tack welds on each stake pocket to rear sill or a full bead where the stake pocket contacts the sill.Before '51, they were just bolted together.

If you're doing a correct restoration on a '51 or later, you need the welds. Otherwise they're really not necessary unless you really plan on loading up the bed.

Bob