From the look of those cross sills in your pic, you have a steel floor bed and wish to convert it to a wood bed floor.
The ends of the steel cross sills are curved to drain water and snow out of the cross sill.
The wood bed cross sills are a totally different design and differ in number.
What you referred to as "bumper is known as the "rear cross sill", and incorporates the rear most mounts for the bed to frame connection.
This link will show you how the steel and wood beds differ
http://www.lmctruck.com/icatalog/cb/full.aspx?Page=14
Note! A wood bed needs to have a way for rain water to drain away or the bed will rot really quickly. Original wood beds from the factory were made of Oak hardwood that was treated for longevity. A solid sheet of ply might be ok if the truck never sees wet weather, but is it worth the risk?
If you want details on a wood bed materials cost, suggest you contact a member of this forum GR8-68 Larry is a good guy and does wonderful work.
