The dimensions are available on the GMCPauls website here:
http://www.gmcpauls.com/47-72_BedWood_Info.htm
If you decide to make it yourself, don't use treated pine lumber. The new treated pine is very corrosive because it uses boron rather than the arsenic used in the old stuff. Fasteners in contact with it (i.e. bed strips and crossmembers) would have to be stainless or hot dipped galvanized to stand a chance in contact with it.
Regards,
Steve Lorio
New Orleans