Mopar made several different flex plates over the years, the original early one were for the 12 inch converter in all V8 that had 6 bolt cranks with 7/16 crank bolts and 5/16 converter bolts,
The Hemi motors had 8 bolt crankshafts and used a 11 inch diameter converter like the Max wedge cars came with and both used the larger 7/16 converter bolts like most aftermarket race converters use today.
I'm not sure of which year it was(1969 or 1970, when Mopar switch to the smaller 11 inch converter with the smaller bolt pattern on the converters. Bottom line is the smaller bolt pattern flex plates have material remove in between the center and the outer mounting converter holes which may or may not help a motor rev up a tiny faster
I have a sack of them on my shelfs in both sizes and some out of date SFI plates that have both converter bolt (12 and 11 inch with both 5/16 and 7/16 holes patterns
B&M, TCI and almost every other race tranny companies stock and sells the one you need for the 11 inch and smaller converters with the larger 7/16 bolt hole sizes.
I have drilled, enlarged several 11 inch small bolt patter flex plates to the larger 7/16 size holes with no bad results that I know of for drag cars
It's your call to drill or replace your flex plate