They probably will but for $40 you can get a new one. Why take the chance? Any used heater core that has sat without antifreeze circulation will rust out in no time. I learned my lesson the hard way.
I agree with Steve. A heater core is one of those jobs that you don't want to be doing again next year because you wanted to save a few bucks (I think I paid $27 for the one in my 68).
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