wheel to wheel width

[Izzy]

I kinda like it as it is - sorta old school box on a newer rig kinda thing
but if your intent - split down the middle and add another layer to widen.

If your going to use a pivoting sub frame then you can use the problem your in to your advantage. Split the bed in two places - about 1" in from the edge on each side and then add a spacer between the cut pieces and the new floor, then you can help hide the the pivot frame . If your using a pivot frame then this is a waste of time.