My source for this was the Retrosheets that are being continually being compiled by the SABR members and are available on eBaseballWorld.
By the way Pete Rose hit .307 lefthanded and .293 righthanded.
AB--Mantle 8102 Chipper 8142
Runs--Mantle 1677 Chipper 1505
Hits--Mantle 2415 Chipper 2490
extra base hits--Mantle 952 Chipper 966
HRs--Mantle 536 Chipper 436
RBI--Mantle 1509 Chipper 1461
BB--Mantle 1733 Chipper 1404
K's--Mantle 1710 Chipper 1278
Bat. Ave--Mantle .298 Chipper .306
OBP--Mantle .421 Chipper .405
Slug.--Mantle .557 Chipper .536
OPS--Mantle .978 Chipper .941
I know you are looking at batting average only, but isn't this a little too simplistic of a conclusion that Chipper is a superior hitter to Mantle when the stats really say otherwise. All Chipper's averages will decline even further the longer he plays and Mantle already has a healthy lead in OBP, Slug Pct. and OPS which tell far more about a hitter than eight points of batting average. They are only separated at this time by 40 official AB's so a great comparison can be made.