I have always said you really can't compare athletes from different eras. Things in the NFL were totally different 60 years ago than they are now. I have had this conversation several times over the years that involve all sports.
If you were to just pluck guys like Jim Brown and Dick Butkus from the 60s and bring them to the present time NFL, neither would be as dominant now as they were then. Today's players are bigger, faster and stronger. You have more guys in the league now who could outrun both of those guys.
Since we are talking theory, I feel the only way a comparison could be made is to have both athletes born around 1995-97 which would put them age-wise at or near their prime in today's game. This way they could grow up in today's world with today's nutrition, medicine, conditioning and of course the added HGH, Steroids and other forms of PEDs.
I think this is the only way you could achieve an accurate comparison. In other words, it can't be done with any real objectivity.