New Idea manure spreader floor

   / New Idea manure spreader floor #11  
I'm in the process of renovating a NI manure spreader. I don't think that the floor we removed was original. It was 1x8, but that meant that some of the boards had only one bolt though the middle. Leaves lots of room for cupping.

The hole spacing in the frame members that the floor bolts to is about 5-3/8" with a few extras at each end. The only lumber that makes sense to use that more or less fits the bolt spacing is 1x12, but that leaves about 3" beteween the bolts and the edges of the boards. Does anyone know what lumber they used?
I have a NI 12 that needs a floor. What did you end up using? Being the bed is over 8' long plywood doesn't work great unless I got 1/2" and used 2 layers.