If you want it to look more original, just use more 6x8 beams to replace the rotten ones. If it was me, I would get some steel channel to replace them. This would be much more expensive at the time of the build, but in the long run, it would be much more durable and cheaper than having to replace the boards every few years. As for the flatbed, I would try to save it because old, original beds to the truck are so hard to come by. Unless you know for %100 sure that it is far too gone, do NOT get rid of it and especially do NOT scrap it! Even if you decide to not keep the bed, or you think its too far gone, atleast put it in the for sale section here on the stovebolt forum! Why scrap that bed for just a couple of bucks when you will probably WAY more in the forum? As for making the bed, it all depends on how durable it needs to be, what its main use would be (Hauling hay, show truck, cattle hauler, all around farm truck, etc.), and what your looking to spend. Please give the fellow bolters more info on what it's prime use would be. Are you looking for cheap and easy, or overbuilt and made to last a life time?