The best year are ones that are easy to get home, in nice shape and a price that you can live with and be happy. If all of that is met, and you have a choice between a few pair, then look at the year.
Replacement parts will not be an issue with any of them.
Construction is also very good on all of them. New ones are MDF, which, if they never get wet (as in flood or soak a bunch of water) is inert and should deaden vibrations very well and never fail. Very expensive speakers are almost all made of MDF (or variants).
As mentioned above baltic birch is very good plywood. That is the stuff you see with many small layers and is used for drawers on expensive cabinets. You will not find it at the Home Depot, it even comes in different dimensions (5' x 5' sheets). Really good plywood.
Last, if I had my choice, all other things being equal, I would look take a pair with A or AA crossovers because they are so cheap to rebuild and considered to be very good. The coil and transformer are really the things that you need. I got lucky with a pair of Belles that had AB x overs, but they had the right inductor and transformer to easily make them into a pair of Type As.
Finally, if you are handy and like to tinker, you will probably wind up changing everything except the bass bin, so keep that in mind too.
EDIT: Just saw your post, go with the 1978's if they are the same price and condition. But if there is a difference and the 1984's are nicer/better priced/ closer, etc., a type A crossover can EASILY be put in the 1984 models.