Here's JC link to blocks and related parts. As I recall 2 blocks are drilled off center, 2 on center.

http://www.oldchevytrucks.com/cart/index.asp?cat2=9515&path=1947+%2D+Early+1955+Truck+Parts%2FBed+Parts%2FBed+Mounting+Blocks+%26+Pads


Drew