Stepside bed supports? Am I missing any?

[GMC"ER]

I am getting ready to start on my wood for my bed. It is a 69 GMC stepside. I had not really looked at it before that closely before now and I am not sure but I think that I may be missing a metal support or two? It had a sheet of aluminum in it and after I took it out getting ready for my wood I just thought that it was missing a support or two. Here is a picture it is kinda hard to see in the picture but I have two of the larger supports in the front part of the bed then with a smaller one then there is another larger one toward the back with nothing in the center. This may be right but not for sure as the truck had been taken apart before I bought it so I have no clue how many or were they belong in the bed. Sorry for the bad picture hope you can see it alright? Thanks for your time, Robert