I had a exhaust stud on my 360 "J" headed motor start leaking. Well long story short the exhaust holes are heli-coiled and I can't get a new heli-coil to "hold" in the head. Re-tapped, new coil with rtv & let setup for a couple days. Install header stud. All seems well till I try to torque down nut. The heli-coil keeps pulling out of the head. Ran the tap in further and got some 1" long coils from McMaster Carr. Same damn thing. I know the threads in the head must be too rounded over to hold the coil in. Question: What is the best epoxy to use here? High temp area that needs to seal from coolant as well.

This is my last attempt before ripping the top end apart. Any ideas would be great.