Hi Kevin,
All the chainguards crack where you described. Here's what I think, not necessarily what is correct. 52 through 55 had the boss drilled for the long chainguard near the pivot point. These did not have the threaded boss on the right hand inside about halfway back on the swingarm. After this my theories get iffy. Some will have the inside threaded boss for the short chainguard but will not have the boss drilled for the long chainguard, others will have both the threaded boss for the short chainguard and the long chainguard boss drilled to accept either. My thought is the swingarms starting in 56 did not have the boss for the long guard drilled and at some point the factory drilled them so it could work for any version, early or late? I went back and looked at your original post, something caught my eye. You say that you bought a frame for a 65-66 XLCH. These two model years will have distinctly different frames. Which year are you doing? Post a picture of the frame you acquired if you can. In particular the area under the seat casting. On another note check the underside of the kickstand boss to see if it is starting to crack as well as inspect the rear downtubes where the enter the top and bottom castings, these points are prone to cracking. The correct swingarm should cost you between $20 and $40 at a swap meet. If you can't find the correct one let this group know, they are plentiful.