You could go Pine if it has been dried correctly (not big box store) or poplar for strength/weight but just be sure to reverse your grain curve and use a tongue & groove type joint. Do you have to use Wood? Composite Decking is a option since your not holding up the world. If you want strength then go Sapele or Epay but the weight might kill your plans. But the bog standard as stated above is oak.
I used oak as well. I used linseed oil and soaked it several times then painted it. It is screwed to the subframe and the screw heads are countersunk quite a bit so no chance of the heads damaging the truck frame. My subframe is spring attached to the truck frame so the screws keep the oak attached to the subframe when the truck frame twists and drops out from under the subframe. The sub frame is extremely rigid so the oak never really sees any movement. Two years and it looks like new.