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A search on this topic produced zero results so here it is...it may be of some value to someone.
I recently removed the boards and rails on my '56 for a upgrade and clean up. The boards are in good shape but the rails have been kept around longer than they should and will be replaced with new stainless. What I discovered was the forward cross member at the base of the front bed panel was a collecting place for water and crud.
I've seen a lot of trucks with nice varnished boards that are discoloring in the forward corners. Apparently, this is due to water sitting forward since most of our trucks are at a slight rake.
A small drain hole forward of the first bolt hole for each rail should take care of it. It makes sense that some of you may not want a opening here but I'm more concerned about water from above than below.
I did that. I noticed the same thing happening and was also concerned. I added a hole at each corner, as close to the corner as was practical to drain the bed. The hole needs to be at least 1/4" dia to drain. 3/8" is much better and allows for crud buildup in the hole.