If you are gonna replace the blocks, do yourself a favor & use 1 1/2 " steel square stock instead of wood on the 2 closest to the cab. You can cut the steel in 3" lenghts & drill a hole in the middle for the carriage bolt.
Its a good idea to use the rubber insulators on boths ends of the steel block. If ya use steel you will never have to worry about the wood block drying out & falling off causing the bed to sag & cracking the rear fenders where they mount to the running board. This will also prevent the bed from resting on the running board to bed splash apron. How many of these have we seen that were chewed up by the weight of the bed slappin' around on it.