I don't recall which of the holes on the tail of the frame are used for the rear cross-sill, but could check on my truck. The rear cross-sill has pieces extending forward, and that should give you the position of the rearmost bed bolt when drilling the wood. The front cross sill is definitely bolted to the frame.The hole for those bolts are directly behind the middle frame cross-member but not behind the cross-member gussets (triangular pieces riveted tot the top flange. The middle cross-sill for the bed wood is the one that isn't bolted to the frame but floats just forward of the frame hump just behind the shock mounts. I have the bed wood instructions from Mar-K and might be able to scan them and post them here. I'll be out working on my truck today and could check for you on bolt positions (assuming that they're the same between '51 and '54.)
