440 lo comp rings worn?
It's been apart if its a 66 block casting and 906 heads. If you honestly don't know what is going on inside or the history, a tear down is all you can do to find out if something is wrong or right. Could be a lot of things to cause the issues you describe.
Have to put a vacuum gauge on it? Done a compression and leak down test?
If it has one of the plastic toothed "silent" timing chain, that could be an issue as they degrade and fall apart.