For a truck like that I would use real wood. The plastic stuff will warp if not supported properly. I don't think the 3 or four cross sills will give you enough support. Plus is will get kind of rubbery in the heat and you'll find that it might look wavey.

Get yourself some oak or other hard wood, put a good exterior finish on it and you won't be sorry. The plastic stuff in a show truck would just look tacky IMHO.