Best thread sealant for exhaust studs into water jacket

If you just needed a "thread sealant" than yes, the Permatex high temp is the best for that.
In regards to temperature for exhaust studs, there are absolutely no issues. This is what the product is
designed for. Besides timing covers, water pumps, thermostat housing, fuel pump, etc.

Now if you have an issue with threads not tightening, or semi stripped,or whatever you want to call it.
Thats a whole other story. Permotex is a sealant, and not a loctight. So if this is what you need to do, some sort of loctight will be in order. I wouldnt give any guarantees on this, but you can try it.
Good luck !