Here are some pictures of my ‘51-53 style bed. Granted it is a 1 ton but it might provide some insight. The 1 ton frame rails are straight to the back (I believe 3/4 are the same). The 1/2 ton rails flare out considerably toward the back as they leave the cab area. I think you might have one of these problems:
1. The board is in the wrong place. Spin it 180 and put it on the left bedside. But if it’s like my one ton it still won’t line up...
2. You have 1/2 ton boards. My pictures of my 1/2 ton, before I disassembled it, show the offset washer holes on the outer board. But I assume your boards fit the box length, so this is unlikely.
3. The supplier drilled 1/2 ton holes in a 3/4 ton length bed wood kit.
If you look at the pictures, the shiny black bed is the one ton. Bed bolts are in a straight line on the 2nd board.
The old rotten, crusty box is the 1/2 ton. You can see that some of bed bolts are on the outside on the outer board. Warning though, this is actually a sheet of plywood the PO put in, so I believe the rear most bolt is absent.https://photos.app.goo.gl/irLgZWSwgmTRVoqJ9
Questions for you:
Do you have pictures of all the wood? Lay it out in order and show us some measurements. If the supplier screwed up drilling ( or heaven forbid, if you did ! 😉), it will be very apparent to 3/4 truckers.
Is your box homemade? Is the cross sill at the rear an actual 3/4 ton cross sill?
Is that 4 1/4” the entire width of the board? If it is, I’m thinking the supplier messed up. But like i said, give us more info before you jump the gun! It’s better to be silent and be thought a fool than to speak a remove all doubt! Get your facts lined up and then decide what to do. This has saved me lots of embarrassment in the past!
Get in touch with Brad Allen on Stovebolt here. He can definitely help you out. If he can’t, he will point you in the right direction. I hope this helps.