my truck sounded similar, but it turned out to be a leaky exhaust gasket where the manifold and pipe meet. (thats 2 with the same suggestion) I would go there long before I started tearing into the engine. I would do a lot of studying and get one of those mechanic stethascope things and do some listening around before I came to any hasty conclusions that involve tearing the engine down. But if you have an original 61 engine, there is a good possibility it needs rebuilt anyway.