This dicussion is apropos. My recently acquired "rust-rod" has very bad wood in the bed. If you step in the wrong place you're quite likely to put you foot right through it.
I plan to replace it soon, but would like to make the new wood look a little less new, so it blends in better with the theme of the truck. I will avoid sealing it like the plague, I think it makes wood look like plastic in any case. Some type of oil/turpentine mix might be the way to go.
What did these trucks come with in the 50's? Certainly not poly-urethane, as that had not been invented yet.
Gustave