Jake0147 is right on. Using a higher tensile strength "harder" bolt will require greater length in tension to be able to hold torque as well, locknut or not. A bolt is generally NOT supposed to be operating in shear, but rather in tension. The clamp load applied is supposed to provide the path for force, or in this case, rotational torque to be transferred... Bolts are sized by the clamp load needed, not the shear strength as a rule.
When I got it, my deuce had all 8 bolts loose, and had for a while. All were grade 8 fine thread with the proper crimp locknut still in place. Out of necessity of not having similar replacements, I used Grade 5 bolts, with the proper long shoulder, and added a lock washer to each side to get an extra 1/4" in tension to help keep them tight. I torqued them to 80 ftlbs per a couple of charts I found online. I messed up the threads a little on the end of each with a hammer to prevent the non-locknuts from getting completely tossed.
After thinking about it some, I decided not to mess with it other than to check them every so often... Fine vs course thread is kinda irrelavent, as the proper torque for each results in the same achieved clamp load. I suppose fine thread would hold torque longer as the nut backed off, which may be why the mil used them. ??
So far so good, with about 500 miles on it. I'll be checking torque again soon, but they still don't budge at 80 ftlbs.
I'm not telling you to go replace your bolts right now, but I am saying that the grade 5 bolts are working on my truck so far, and they're holding torque without locknuts or locktite.
C