My OEM sealed ones have 300k on them. There's a drive line shop here that won't use greaseable. I don't know what's changed over the years but I'm seeing much better results with sealed.
That oring is what gm used on a lot of engines. Not sure on their reasoning. I believe to use the other style the guides have to be machined to be able to have them snap on and stay .
I wouldn't be surprised if both block and head are original to the burb. If those are big valve heads I wouldn't get carried away with power. Power usually means heat and those big valve heads were notorious for cracking between the valves