I would keep them for now, but not attempt to fill in the holes by card-by-card by haggling with dealers at shows or on ebay, etc. The way 1970s stuff is coming out of the wordwork now, it should be pretty easy to purchase large lots or entire collections and then fill in the holes with one shot from the bulk you have accumulated. After that, sell-off of the excess dups and what you do not want, and fill in the sets basically for free.
Once the sets are complete you can sell them then, as they will bring a much better price.
Good luck!
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