If the truck isn't going to be parked outside or see a lot of sun (UV is a killer to clear wood finishes) then you can use regular exterior-type wood finishes. If this needs to stay looking good through all weather, I'd suggest asking on a board where they work on "woodies". I suspect it will be some kind of catalyzed varnish that runs $200/gal.
Ordinary wood would get an exterior sanding sealer and a gray glaze. I am not sure I've ever seen "exterior" glaze per se, but you could try an oil paint wiped off with a squeegee, touched up with some 400-grit paper when fully cured, then clearcoated with lacquer.
Casey

Casey, Thanks again!The truck is a 1958 Chevy. Cameo Carrier PickUp. Total production was 1400 units. Everything associated on the bed is what makes it rare and very special. I am not building a show truck to win national awards, just a driver that will be well taken care of and passed on. At 73 I don't have the experience with wood finishing and really appreciate the help. I need to do it right the first time. Therefore, what has been suggested, ironizing to turn the boards black and yet see the grain, wiping grey glaze into the grain and then wiping off across grain and then sealing with a product that will withstand all weather conditions will result in the finish I'm looking for. Right?

Jim

""........withstand all weather conditions will result in the finish I'm looking for. Right? ""

.....35 years ago I had a Van with laminated wood running boards and other wooden details........the finish was always a problem......

Lightweight fiberglass cloth and epoxy resin finally saved the running boards......

Good Luck

The fiberglass cloth idea is possible! You'd need to make sure that the resin is compatible with the glaze; this kind of thing is most often done over bare wood. It would be compatible with the iron stain alone. The glaze would be the tricky bit, wrong kind and the resin would peel off.
Whatever clearcoat you use, it has to be compatible with the glaze coat- so it might be smarter to wipe on a gray lacquer as the glaze coat, so you'd be assured of a good bond. Any excess glaze could be removed with thinner.
Good luck. I have seen several cameo's, not sure what date.
Casey

Casey and abnorm, the idea of the fiberglass cloth won't work because of the groves to be routed on the top edge of the boards to accept the polished s.s. runners. I'll stick with the iron stain and grey glaze and consider a marine polyurethane top coat. Any other advice would be appreciated. Thanks, Jim

Fiberglass wont be a good idea for me too.. Rubber is quite good though its not really good for the heart..

Here is a link that might be useful: Truck Bed Rack Mount Kit