Wally,
I would replace the bunks with treated 2 x 6's. The same as what you are 
replacing. The bow in the bunks comes from having the boat on the trailer and 
the new timbers will get the bow after a short while. When attaching the 
timbers, you will notice that there is some play in the bolt holes, allowing 
for the stretching and eventual bow. When taking off the old bunks, mark them 
as to location and which way they face. Use the old bunks to mark and drill 
new bolt holes and cut for length. 
I found reasonably priced bunk carpet which is UV and mildew protected at 
Lowe's Home Improvement store. I purchased stainless steel staples, but used 
mostly aluminum roofing nails to attach the carpet to the bunks. Attach the 
carpet prior to installing the bunks. I would also recommend replacing the 
bolts, nuts and washers with new as they are inexpensive. I used an air 
wrench to help with the removal and installation of the new bolts, otherwise 
it would have been a lot of manual labor.
It's not a hard project, but a little planning and forethought will give you 
a nice looking job. One of the trickiest areas are doing the ends of the 
boards with carpet. Look carefully at how they are currently done and try to 
place all fasteners (nails or staples) on the bottom edge of the bunks, out 
of site and won't cause any problems scratching the hull.

Rummy