since you are building one for the street and not the trail its gonna be different from most of the flatbeds on here. You can use wood in the construction of it, but the frame work needs to be steel. I would make the entire thing out steel if I was making a flat bed.
Or go to the junkyard and get another pickup bed for the truck that isn't rotted away. thats the easiest way.
But this thread is just a chance to pimp some photos, so here goes. mine is made out of 11 gauge steel. it weighs 90+ lbs.