Russell boxed out Wilt and Nate Thurmond who were the same size as many current centers. Larry guarded athletic forwards like Wilkins and Nance in his day. Plus you have to take into account players from yesteryear would have the advantage of the nutrition and workout programs today's players do. They'd also have access to video to study their opponents. Think of what a Basketball mind like Bird and Magic would do with that. Plus as others have said the older players knew how to play the game. Some current greats like Kobe, Lebron, Melo etc struggled with pick and roll d and how to shoot from the outside.
I wasn't talking about how they would do with the advantages of today's players, I was talking if you literally took them in their prime and put them in today's NBA.
Also, Larry was not a lock down defender. He was sneaky, and clever and had good hands, but he was not a shut down defender. And I certainly do not think he could guard Lebron.
Bird and Magic didn't have video tape? they were using film in high school football in the 1960's.
Not trying to diminish their greatness, but I think that they would have trouble with players in todays NBA just like today's NBA players would have difficulty stopping a 6'9" point guard in transition, or stopping a 6'10" SF who can shoot from anywhere with either hand. I am convinced though, that in a championship game, both in their prime, Russell could not handle Shaq. Just flat couldn't.