This is an interesting thread with lots of room for thought. I am not an engineer and claim no expertise. But, I do think there are many variables. I used to race motorcycles, and when building high out put engines, the torque placed on threads was CRITICAL. Some nuts, under a great deal of stress, required only a few foot pounds of torque to achieve maximum strength, and they usually only had 3-4 threads engaged. Too much torque gave guaranteed failure. That is for you engineer types to figure, I just followed the specs religiously. Personally, I would prefer shorter plug, say 1/2 inch, properly torqued, to one even longer that has been way over torqued. You lose much of the strength of the threads if you are tightening them up to the point you need a 24 inch breaker bar to tighten them. It might be helpful if someone had some evidence based data on this for breechplugs.
Another variable, which does not address the OP's question, is, in my opinion, we use a lot more powder than they did in the old days. People today routinely shoot 100gr+ loads in 50 calibers, I don't think those were normal loads back in the day.
