Herculiner is a great product for waterproofing wood, and it would look pretty good IMO with your black cab. What I ended up using for my wood bed was Spar Urethane high gloss. This gave it a clear coat look that beaded water when it rained.
here is a pic of the wheel rub when it was just starting, before the blow-out.
Here is a pic of the wood all shiny, still after being used to move around a full truckload of firewood.
here's the wood