Is the metal dowel epoxy'ed in? I had a problem with a somewhat similar design, using a threaded barrel nut and bolt. The hole up the neck weakened the neck and I had two crack across the heel, with the grain (both after some abuse). I have gone to using an oak dowl where your metal dowels are, and then put a brass insert in with the threads of the insert threaded into the cross grain of the oak dowel rather than just the end-grain of the neck itself. I tested this with a neck mock-up. Dropped a 3 lb sledge onto the headstock and it just bounced off.