That does appear to be a 54-55 bed. I've seen those trucks with those style fenders too often and in my opinion it just doesn't look right. I don't blame you for wanting to change. Rear fenders for these trucks can be hard to come by though, unless you're fine with fiberglass.
Checking the price of the individual pieces from a vendor isn't the best indication of the resale value of a used box. I've seen a box with fenders for sale for $600 up to $2000, depending on the condition.
