Treated SYP is still the best for a rugged and inexpensive truck body floor for a flatbed. Grigg's suggestion of quarter sawn oak surely must be as a pickup body floor. Thats what they make furniture from. SYP is hard, sturdy, cheap and available. Used motor oil can even be put on it as a preservative tho' I'm not suggesting it. It is the material still used by nearly all the major body builders that provide wooden floors. It can also be had in shiplap or T&G.
