Hey Jim,
If you go with the 3 3/8 by 5 1/4 from Mo's sheet it will do just as advertised (sharp breakpoint). But the 4 1/2 by 5 1/4 should give you more length and snap (that is how I am going to drill mine). This is from the morich drill request form for pin placement.
3-3/8" Maximum Flare (Maximum Overall Hook)
4-1/2" Maximum Flare (Slightly less flare in oil; Slightly more in dry)