replacing bunk boards

I'm going to re-carpet the bunks on the trailer. I figure to do the job right, I'll replace the boards and bolts too.

Question #1; what type of wood is used for the bunk boards and what size? I know the length.

Question #2; do you seal the wood with a marine varnish prior to carpeting the boards? Or is that making the job much more difficult than it has to be?

I already have the carpet and staple gun. All I need is some wood.

I don't want to use Ned02's style of installation though. I don't need to be upside the head while installing the boards.