Wood was used to allow for frame flexing...steel will rip things apart over time. Find a local hard wood that is cheap to buy, cut to size and paint it with POR-15 on all 6 sides, two coats and it will out late you.
Most truck bodies were custom made so there is no right or wrong way to do it, just build it so it meets your needs...don't overthink it! In the end no one will ever question what species of wood you used.
My .02 cents!
Mike B
