Here is your answer from the JB Weld site

JB Weld FAQ

Basically it says JB Weld will hold up to 500 Degrees F. Continuous temp, 600 intermittent.Not recommended for exhaust manifolds, but I'm thinking the head itself won't get that hot.

They also have a high heat product they recommend for exhaust manifolds:

high-heat

That would probably work.

Kurt