Question for the OP -- How will you be using the truck? Do you plan on actually hauling stuff in the bed (thus needing a more durable finish) or are you wanting to show off the beautiful original (old, tight grained southern pine) wood in all its aged splendor?
If you want it to look as original, the durable finishes as described above are good.
If you want to show off the bed wood (but still use it from time to time), I like Otto's suggestion of linseed oil and turpentine. I've used that to finish some oak pieces I've made and it really makes the grain pop while giving a good durable finish that glows. Also gives it sort of an antique look. I've also used a rubbed polyurethane and that looks nice, too. The linseed/turp finish is more old school, though.
If you REALLY want to show off the wood, and highlight the patina of the smears of original finish, you could go all out Old School and do a good sanding job (by hand entirely) and then finish with about 10-20 coats of spar varnish (hand sanded between each coat). But that route is not recommended ... (you'll hate me). unless you are actually insane.
Some of the other wood workers may have other ideas, too.
I think, though, the real questions are how you are going to use the bed and how original do you want it to look.
Last thought -- the bedwood in my '49 is original and I keep it up with used motor oil mixed with a little diesel fuel. Keeps it nice and black!