yoshi, Will try and give you some more info about the bed wood. Hope this helps.
Your eight boards should be 47-1/2" as shown on attachment you showed above. With room for 1/4" carriage bolts thru the strips, with approx 5/16" between the boards, this will give you 50", which is the bed width.
The first pic shows my bed (not a good pic) starting from the left side with 4-1/4" board under the angle against the bed, then a 7-1/4" board, then 5" board, then 7-1/4" board and another 7-1/4" board, the 5"board, the 7-1/4" board and last 4-1/4" board under the angle against the bed.
The second pic is of another truck, but here you can see the grooves in the wood boards where the strips should lay. This helps to keep the boards in line for spacing.
I also posted on another thread about how to bolt the bed to frame. How the bolts go thru wood, cross sills, blocks and frame. You can also read that which may help. Here's the link
http://67-72chevytrucks.com/vboard/s...d.php?t=667704. Hope this helps..Jim