Okay I just walked back out because I'd forgotten to measure the tailgates while I was out earlier.
The '54 tailgate is 51 inches across the top rolled part. It appears to fit into the end of the bed about 1/2-inch on each side. This is what yours should look like, unless we are all wrong about what it is.
The early tailgate I have probably came off a '49 but I say that only because the guy I got it from is working on a '49 and I think it is a reject off that truck. I also think '47 to '53 tailgates are either all the same or close enough for this purpose.
It is only 50 inches wide across that some roll, but it appears it has a lip that would make it at least 51, maybe 52 inches wide across the rear face.
My conclusion is that while you can not just swap the two, with a fab shop available it would not be impossible to make the early fit a later box. It does not appear to be a simple bolt on swap, but rather something not a big deal to someone with some cutting tools and a welder and some additional steel available.
