Exhaust manifold stud issue

[Oldmactech]

Doin a head gasket and in removing the exhaust manifolds every one of the studs came out of the head. I’m about to reinstall the head and manifolds and have been soaking the studs in PB Blaster to get the nuts off (at least some of them).

I pinched one in a set of vice grips and used a bit of sandpaper to get a better grip but still wound up slightly galling the threads to get the nut loose. Need a good tip on getting a good grip on the stud o I can twist the nuts off without messing up the threads.

Thanks.