update's update - thanks for the advice about not trying to pull one head, bec that'd been terribly frustrating even if it had went 'well'. one cracked valve and one distorted. fortunately, i had a completely machined one in the basement and only needed a manifold gasket. and in the process, the exhaust donut gaskets completely disintegrated upon 'removal'. do they naturally disintegrate with age, or does that indicate a leak that i wasn't hearing?