[C. Ludwig]

The manifold and gasket will not hold it in place. The gasket actually seals around the outside of the insert so there is nothing that's going to hold the insert, even after the manifold is tightened.

Drilling the insert is a great idea except that you need to get the whole thing out to do that. Without breaking down the engine it's going to be hard to get the sleeve out.

Had a customer bring me an engine with the exact same thing you have. For a quick fix I built up some weld on the manifold flange and filed it down until I got a snug fit against the sleeve to hold it in place and that worked well. His was bad enough you could hear it rattle around when the engine was idling.