I'd paint the wood black or if its dark, the body color. Black will be invisible and easily touched up. I think the truck in your avatar or whatever would have been black, while later, or at least around '73 body color.
I'd fear bedliner will fail to adhere over time, and plywood will fail, as well as look like plywood.
Also, if its a stepside, making oak planks is even easier than a fleet. Plain steel strips are cheap.
I think you'll find it pretty easy to snag templates for cutting the planks here, or on the Mar-k site:
www.mar-k.com
Oak will last 40 years and look good doing it.
If you have a woodworker friend or have a tablesaw and router you can get white oak (I think) from a wood supply place, maybe let it dry a bit, trim to width, cut the grooves and trim to length, and apply a finish (not bedliner).
How much will the truck spend out in the weather?