69 250 exhaust leak

[Old Glory]

Started my old 69 after sitting for 12 years and need to address a problematic exhaust leak between the manifold and exhaust pipe. As best I can remember I would tighten down the two bolts and after a short period of time, they would simply come loose and the exhaust pipe would slide down the bolts enough to cause a leak.

Any tips on how I can secure the connection? Are there special bolts and washers? Loctite?

Thanks