when I was at the same point as frankthetank and started assessing what was left of bed sills, what they cost to replace, how hard the rear one would be to make I did this:
cost less than the varnish for a wood floor.
Use any full size truck box floor. It was dead easy, took maybe a weekend to complete. I can use my truck as a truck.
I have looked after varnished wood outdoors before, zero regrets to not do so again. But I do understand the attraction of a spiffy wood floor too.
I can take better pictures, explain how to trim up the bed floor if someone wants to know