How can I close this?

Hi - New to MoI and looking for assistance.

I have a hole that I want closed. Basically I would like to select the vertical profile on either side and just "slide" the gap closed.

I have been able to do so ( at least visually), and end up with a single mesh. However I don't think I'm ending up with legal geo, as when I do a boolean difference, the resulting mesh is open again.

Any tips? Thanks.