EGT's, right out of the engine, are around 1500-1700*F, most front O2 sensors are close enough to see 1300-1500 deg. So your epoxy must be able withstand that. Mufflers rarely get above 500-600*F.

Then, there you pulses from individual cylinders, that just adds to the "pressure".

Ether find a Helicoil insert of proper size (can be very expensive), or just replace the manifold.