Best way to remove Crankshft Plugs

[Pete from OZ]

Hi Fellas,
This is my first Flat Head build ( 8BA ) and I would like to know the best way to remove the 4 plugs from the Crankshaft to clean out the Oil passage,

They are a Slotted type. I have tried my Impact Screw Driver. But no good.
I also have to remove the Oil Gallery plug in the front of the Block. Same slotted type.

Any help would be much appreciated.
Thanks,
Pete from OZ.