There are tools designed to grip the stud away from the threads, or you can just put two nuts opposing each other onto the top threads.
Find a chart that gives torque spec by bolt size, there's one here in the torque spec sticky thread, I think. Tight enough to not vibrate loose is the spec I would use, the actual clamping of the head gasket will happen when the nuts at the top of the stud are torqued to spec.
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