How to repair valve guide boss

That happens a lot installing new guides. When I was doing head work and installed new guides, I milled the top of the old guides OFF. It's not a "guide boss" but just the top of the factory guide. There's absolutely ZERO need for it to be there. You can mill them off with the appropriate guide cutter (whatever the OD of the new guides are) and run a positive seal that will install tightly around the new guides.

That's how "I" would do it.