That unusual concavity that is shown in the picture is in the vicinity of the intake valve seat.

The seat is obviously not falling out or anything.

It certainly had nothing to do with head gasket seepage.

The intake valves & guides are generally not a problem on these heads.

The finish in the region of the concavity looks like that feature/defect may have been there since the beginning; shocking as such a notion may seem.

I would suggest first checking the head for flatness. Even a rough eyeballing on a surface plate or with a good machinist's straightedge would give you a pretty good idea of whether you've got 0.010" out of flat or 0.030" out of flat.

Then measure the overall thickness of the head and check this against the spec for thickness. This will give you a pretty good idea how much material has been taken off the head.

If these two aspects of the head look good, THEN take all the valves out and see what you're in for on the valve job.

My experience has been that you're probably in for all new exhaust guides, but hopefully you won't need any valves; those are pricey.