Mine is a 1951 also, 7 strip/8 board.
I just finished fitting all new boards (which I did myself, as opposed to buying the boards already done), and bought new stainless strips and angles. The strips use 1/4" carriage. The angles use 5/16" carriage. I'm sure of this because I installed the angles on the side of the bed first, (the old angles were spot-welded, the new ones I drilled and bolted on). I did buy the stainless bolt kits for the bed to frame installation, and stainless bolt kit for the wood to bed installation, and I know for sure these are the sizes. I purchased all this stuff from LMC, (I should have shares of stock in them by now), and all of it is correct and fits. Appears if you are saying that a 5/16" carriage bolt will not fit your angles, then something is wrong.
Mike
