Cujo8,
there is definitely something amiss with all the bolt problems. The very early Chev engines, maybe in the 20s had problems with the steel and bolts could not be reused like today. Your engine is much too modern for that. Age does not cause problems like that. I would suspect that they were over torqued by a previous owner and that would fit well with the cracked cap. The other possibilty is that your torque wrench is bad, really bad.
You gave a perfect description of a stretched bolt, almost a textbook description! I was reading an Engineering textbook on bolted joints a while ago and would you believe that the force is all taken up by the first few threads and all the rest are just along for the ride. To actually use more of the threads, the bolt and the joint have to be designed to distort, which these are not.
