I think I'd just buy an early bed if it were me. I say this in part because it was my first thought when reading of your problem, and in part because yesterday I did buy an early bed rather than deal with the issues adapting a late bed to an early frame.
I know the depth is different between the two boxes, I'm not sure the width is the same. If the width is different swapping tailgates is a problem.
I'm about to go out to my shop and will measure the width of the tailgates, but it may be some hours before I return to the computer with the results.
