I remember seeing a TV feature with Rick Barry. He was with Artis Gilmore, at the Hall of Fame. Gilmore was at the end of his long career, and Barry had been out of the game for a bit. They were shooting at a rim, while talking. Barry, one of the best shooters of all time, barely made any shots, but active NBA player Gilmore made like all of his.
Gilmore played at a time where players would try to get as close to the rim before shooting. If he was coming up today, I wouldn't be surprised if he could step out and shoot the long ball. I saw Kevin McHale as a summer camp right before he started taking any threes, probably around 1989-1990. He was shooting threes from the top off the key, and made 20 in a row.
Back in the day, NBA greats became good mid-range shooters through repetition. No reason to think they wouldn't have been able to shoot the three if they trained for it. Also, with the no hand checking rules we have today, and with the threat of the threeball, imagine how unstoppable some of them would be, like Tiny, or Maravich.
One last story about how old time basketball worked. Sidney Wicks went to John Wooden and asked if he could take more outside shots, probably figuring it would help his NBA stock. Wooden told him to take ten shots from the deep corner, and if he could make all ten, he could shoot from there in games. Wicks made nine. Wooden told him that he could make 10 for 10 right near the basket, and that's what he was going to be shooting in gmes.