People are constantly asking about here about what brand headers will fit right. The answer is, was, and will always be the same; cheap, mass produced headers may or may not fit right. Unfortunately that's just the way it is. Mass produced headers are simply not made to exacting specs every time so where one set will fit like a glove, another set of the same brand won't fit for crap. For Olds, American Racing headers are the only ones that will fit right every time.
77-80 year 350's are all identical. I don't know if there's a way to determine the exact year but it doesn't really matter since they're all the same. They are all low performance 'smog' engines in stock form. Of all the year Olds 350's, they are the worst but are still better than any 307.
