I have used in the past a liquid product I think from Loctite on my Harley's to fix bad threads. What I am up against is the threads on my Roof overhead Model BB, that the exhaust manifold attaches to by four bolts are very loose. I had one that pulled the threads, not to the point they are gone but as I was tighting the bolt it didn't feel right. I can pull the head and drill and tap out to the next size. Have you had any luck with a liquid thread fixer in high heat applications. Bear in mind that the bolts go through the manifold and are exposed to the exhaust gases.

helicoils would allow you to use your original bolts. coat the threads with neversieze. It is outstanding extream high tempiture antisieze.
Scott