I guess I didn't take very good pictures when dismantling my 1962 292, because I can't find out what bolts into these holes in the heads.
empty - a set on Flickr
They are all the way to the back and not covered by the intake. For now, I just plugged them with short bolts, but I was thinking that at least one of them would be used for a ground wire. Is that correct, or is there another purpose?

 

If you look at the heads they can be installed on either side. If you had swapped heads that bolt hole would correspond to the front intake bolt hole by the water passage.

That said you did the right thing by plugging them or oil would come out of them when you start the engine. They are open to the back pushrod passages as you can see in your first photo.

Just be sure they are short (~1/2") because if too long could hit the pushrod and bend it.

I believe the plug wire loom bolts to the passenger side one. The drivers side I do have a body ground bolted there.

 

Thanks for your answer. Pretty much what I figured, but just wanted to verify with others. I used 3/4 in bolts, and they look like they clear by the time you add a washer and maybe another attachment in there.

 

Thats so funny, just found the same two holes on mine and was wondering what they were for.