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.
I don't like the striker pin to be used for height adjustment of the door only in and out. You should be able to adjust the rear height with the hinges as well