Anybody use these holes in the end of the head for attaching an engine sling? I’d prefer not the attach to the manifold/manifold bolts so was thinking to make a couple brackets that would attach to these holes and extend above the valve cover.

You can use them just fine, but just don't have too long of a bolt in there when installing on firewall end of engine. Some bolts won't have enough room to come out .

I have used them before. Worked well

All the time!!!!

Early 60 engines used to come with brackets bolted there.

Those will work fine. I also make lifting tabs that bolt to the exhaust ports.

http://arengineering.com/products/lifting-tabs-big-block/