Me being me, I'd use quality (read: "not Chinese") socket head capscrews with LocTite #242, or equivalent (I've never liked split lockwashers.)
You may use a hex head capscrew if you are able to get a socket on there reliably (driveline screws should be properly torqued,) and the idea behind using a 12-point is a smaler head, which gives you more wrench clearance.
The nice thing about internal-wrenching heads (socket heads) is that the wrench can be smaller than the screw head. Also, if you use hex sockets, you can find "ball end" hex drive bits that will allow entry at a slight angle - typically up to 25-30* off-axis - while still being able to torque reliably.
If there is any possibility that the screws you're looking at came from China, find another vendor. It has been my long experience that Chinese hardware may be safely considered "counterfeit" (I've run across Chinese steel screws that were supposedly SAE8, but the threads pulled off of them with less torque applied than on an aluminum screw of the same size. I've also had Chinese SAE5 screws strip their threads running them into castings - which is backwards.)
Whatever size/style of screw you use, just make sure it's a good part (ABC - Anywhere But China) and consider the use of LocTite #242 to be mandatory. Even when I've deformed threads on screws, I'll still use LocTite for the driveline.
(NB: If a socket head capscrew is properly made, it will be H&T to SAE8 or ISO 10.9 by default.)
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