The late grp'a'/wrc modification was done to the block because of clearance issues with the FFD 8.5" diff, the blocks are counter bored dowels fitted in opposite 2 corners and taken out to M10 x 1.5, we use a combination of studs and cap screws to make it easier to install, but unless you are planning to run the 8.5" diff i'd say the 12mm mod to the bolck was un necessary....