Hypothetical question, Burt and Jim Brown were the same size. Lancaster did play football when protective equipment was laughable.Absolutely unlikely that Burt would have tackled Jim Brown. Burt was born in 1913 and would have been about 50 when Jim Brown started playing.
Here's an interesting story, John Wayne was a legit tough guy and a mean drunk. He asked Burt Lancaster to play Liberty Valance, Wayne's logic was that having Lancaster on set would keep him from drinking. Burt rarely drank, during the filming of From Here to Eternity, Frank Sinatra and Montgomery Cliff would go on alcoholic benders - they bought Burt along for protection. It was said that he would fireman carry both actors back to the lot when they were to drunk to stand.