Cabinfever (pretend you're a little girl) has a good way of saying don't overtighten your bolts. Here''s a good rule of thumb: Tighten until the lockwashers are flat then apply just a little more torque. If that doesn't keep things tight you have other issues. Double nuts is an option but another is to replace the studs and nuts with good, grade 8, Allen head cap screws. You probably won't overtorque them with an Allen wrench and they will hold better than studs and nuts. If your mounts don't fit the frame then you need to adjust for that. A good solid engine mount will go a long way in eliminating vibration and loosening of fasteners.
Tom

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Age and Treachery Will Always Triumph
Over Youth and Skill