When I make my "Legends" series of MLB Showdown customs*, I have to be careful how to scrutinize the data.
1) Dead-ball era and 1960s era hitters are a pain to try and do. Just do the best you can, and I usually up the offense a little bit, imagining if they played in today's era.
2) Peaks matter. I usually take a stretch of time when the player was at the top of their game (as judged by OPS+ or ERA+ as a quick reference), not taking into account decline because frankly you remember those players at their best, don't you?
3) Take the down with the bad, don't cherry-pick. This isn't usually a problem, but players like Roy Campanella were up and down. Just smooth out the data.
4) Make an average of the peak (min. 5 seasons I prefer) and extrapolate from there.
*It's got Hall of Famers and non-Hall of Famers, so the blanket term for cards for retired players is "Legends".