If you are trying to do this with as little work as possible, just throw the plywood over the top of what is there. If you have to take it all apart to put the boards in, and you are just using the truck for a driver or a "not for show", just get some 1 by's and cut them to fit. I believe the instructions can be found here at the bolt or at Mar-K (look in the upper left of this page). I just used some good construction lumber and used a good UV resistant deck finish on mine. Replacing the boards as they were from the factory will eliminate a ton of squeaks and rattles too. http://www.mar-k.com/bedwooddimensions.pdf