I am building a bed for my trailer, which is now just a frame and axle.

the frame rails are 43" outside to outside and my bed width is gonna be about 7'6" with an length of about 13'. I am gonna haul my rig which weighs in at about 4000lbs fully loaded.

What does anyone recommend for cross support? 1"x1" x 1/4" angle? or something more? its gonna sit at about 48" high as it is so I really dont' want to use channel or I beam but I could if I had to.

I was thinking about running 12" x 13' planks. would this be ok if I had enough cross members?