Best thread sealant for exhaust studs into water jacket

Yeah - I use liquid Teflon for oil and coolant, but I wouldn't use it directly adjacent to the exhaust port. I doubt it will get past 600* given coolant is surrounding it on 3 sides - but everything has it's limits.
Sorry on the size I stated being wrong.
Not sure which port it is but I've done this in the chassis many times. Loosen the mounts, use an angle drill & the socket type tap driver. I just think relying on sealants and lockers to retain the stud in worn threads is temporary. But it's your car.