What you need is 2x6 shiplap #2YP. It should screw into the steel crossmembers assuming they are steel. Buy pressure treated materail if possible otherwise motor oil is a decent substitute unless it's for show but were talking work truck here. Use self tapping 3/16 x 2 torx head screws especially made for this use. The screws are available at any fastener supply along with the driver bits, get several and put them in with an air impact, drilling the wood before inserting the screw. Clamp them into the floor area but allow a little room between ea for expansion just the same as you'd do on a deck. The outer frame and crossmembers are likely one welded assembly and shouldn't present a problem. None of that needs to be disturbed assuming that is what you have like a modern platfrom body would be constructed. You can chisel or unscrew the fasteners used on the floor now. I always torched them off because it was fast and effective. Don't leave the side pieces open but tuck them under the steel perimeter by routing the materail away and use only full length lumber, i.e, no splice joints. Cut the length approx. 1/4-3/8" short depending on the bed length to allow for linear expansion. If you put the lumber in too tightly it will pop all your screws and even twist steel crossmembers. It is a pretty straight forward job and you shouldn't have any trouble.
