OT: What type wood between frame rails and truck box?

[gary678*]

Black Locust is the way to go if you can find it. Naturally rot resistant, no chemical treatment needed. I used it for fence posts and vineyard posts. The below ground part showed only a little deterioration after 25 years, the above ground part was basically 100% intact and I guess would last 50 years or more. Shagbark hickory and oak are also good with oak being the easiest to work of the three. Good luck!