I used 2 x 12's on mine, with 2 x 4 uprights in the stake pockets. There are pics in my gallery postings. The headboard on the '59 is 3/4" tongue and groove car siding. The one on the '47 is heavier than that, possibly 5/4". It was part of the original grain box.
I see no reason why 5/4" would not work for yours. Mine were definitely overkill!
