Mopar made three different 6 bolts for the crankshafts stock production flex plates, 12.0 inch with the 5/16 bolt and a 11.0 with the same bolts as well as a 11.0 with the lager 7/16 bolts for the Hi Po motors like the 426 M.W and 440 magnums with the smaller 11.0 inch Hi Po converters.
A lot of the early, pre1969 I think, Hi Po 375 HP 440 motors came with the 12.0 inch converters with the smaller 5/16 converter bolts also