Valve stem seal question

I plan on replacing the valve stem seals on my 351w. The question I have, I don't know what ones to buy haha. I have no idea what year the heads are, or how to find out. Or if it matters what year to get if they are all the same? They have the larger spark plug if that helps. Can I just buy any 351w valve stem seal or did they change through out the years? Thanks.

I pulled one of the old ones off. Its just a rubber sleeve with a spring on top that is suppose to hold it in place but does a horrible job. The seal moves up and down the valve very easily. I have a new set of felpro ones coming tomorrow morning. Hopefully this will fix my problem.