I just measured my old floor boards as I am also replacing the bedwood on my short bed '51 3100.
I have two cross sills.
Measuring from the front of the bed, the first cross brace goes at ~27" and the second one is at ~ 54" where the holes are in angle strip.
Mine was rebuild by the PO but, it seems to right.
Sorry, I didn't take enough pictures during disassembly. Note to self, take more pictures.
If I can figure out how I will post the two pictures I have.
