I've made a blockoff plate to remove the waterneck from my 99 head. Should I cut a groove for an o-ring, or just use some gray sealant?

It seems like it might be overkill to add o-ring, but the waterneck has an o-ring.

 

If you have the ability to make a groove, I'd do it.

 

Yeah I do, since I'm tapping it anyway for a water port I can use a hole saw.