Five years ago, I bought some steel stock and gasket material to make block-off plates (and gaskets), for about 20 block-off plate kits - but the project got deeply buried in my garage - so something to do on another day (or year...).
You know I never even thought about just changing the exhaust manifold gaskets or just tapping and plugging. I guess the early 70s or a 90s gaskets should block off the holes in the heads if not