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
I've had pretty good luck with just a high quality putty knife, a thick one that's not flexible. I sharpen it every so often and have even used it on aluminum parts. Key is to be careful. I'd shy away from the carbide one.