The Cubs PR team put some actual thought into selecting the players included in the 82 card set. Players weren't just randomly chosen. There was a connection for each card.
For example, Starlin Castro, #13, was put onto the card for game #13.
Shawon Dunston, #12, was on the #12th game, against the Giants, a team he also played with.
Dave Kingman was included on the card for game #21, on May 14, because on May 14, 1978, he hit three home runs against the Dodgers.
All 82 cards were thought through in the same way.
And how do I know?
Because the Cubs also printed up a two-page sheet that gives the details on all 82 cards. That was a very nice touch. Even nicer, the seller I bought my set from included the sheets, and I've got them here below.
You should be able to click on the pages to get a better look at them in a new tab (if I got my HTML correct...if not...sorry, I tried!).