I have a few thoughts, having worked with wood quite a bit. We all know that the original floorboards were flat sawn yellow pine (or something like that, eh, Tim?) Myself, considering that all finishes will eventually fail, recommend using quarter sawn wood of any variety to avoid the cracking that always appears in flat sawn wood. White oak is a sturdy and reasonably common wood, so quarter sawn white oak is a great alternative. The finish is up to you, myself I don't like the idea of encasing fine wood in plastic, which is what modern finishes are.
I have had an idea, though, if anyone has good connections with a saw mill. The first log from the tree, especially the end that is at ground level, is made up of lumber that is so permeated with resin that it is nigh impossible to get any finish to stick to it. This is the lumber that was traditionally used for wondows and door frames, largely because it is more water resistant than treated lumber. We get boards of fir in the shop that ooze resin. A bed, or side boards, or any weather exposed application of wood, made of this sort of lumber might be fantastically weather resistant even without any finish.
But I have not tried it yet. My own bed could use replacing, but first it has to run better...
Karl
