The only question then is how much force is required to obtain the amount of friction required to achieve the desired result. My past experience is that a 3/8 SS nut and bolt will do the job. There are no guarantees that it won't rust but the reality is that it is much less likely to do so and hence, come apart more easily when needed. My point here is that to change to a hardened bolt will cause you a huge headache down the line if (and considering the environment it lives in, that's not a big if) it ever rusts and the threads do go south. Cutting off a hardened bolt up side down in the bilge with a Dremel Tool is never fun. My transmission is a Hurth HBW10 connected to a Perkins 4-108 and while it is tight, there is room for the proper size lock washer nut/bolt combination if you get the proper length bolt. The combination leaves me with one and a half threads past the nut. I guess it's all in what you're comfortable with. I'm not saying that my way is the best way, only that it has worked for me for a long time with not a lick of trouble. I'm always able to get my coupling apart, it has never failed and I never have to deal with rust. Once again, this is all just MHO.