Here is a subject that even I have some experience with. My own truck has side boards that are about as old as the truck, the truck also has as the original bed. The side boards are pine, painted green to match the truck. They are as high as the bed, look kind of goofy, but I sure can load a lot of firewood or lawn scrap.
I work with a lot of wood, mostly pine and white oak. As a rule, wood will last longer of it is more slowly grown and has tight rings. As a woodworker, I always prefer quarter-sawn lumber because it is more stable, but neither the original bed or the wood sides are quarter-sawn on my truck. Somebody mentioned the problems with pitch/resin in pine. I would point out that old woodworkers, when making windows and doors - other applications where wood tends to rot - prefer to use pine which is rich in resin. The lowest part of the tree - by the root - has the highest proportion of resin, and lumber from this part of the tree can be extremely rot resistant. When you see resin running from the end of boards, you can be sure that this wood will last.
It is however not so easy to get this wood to hold a finish. And since in my branch I never have to deal with such finishes as are mentioned on this page, I'll leave that subject alone. I can say that while my old bed is pretty worn, the bed sides are in excellent shape. I am sure they haven't been painted in over 35 years, probably a lot longer. I think you will not have to worry about the finish on vertical bed sides like we all need to worry about the floor.
Karl
