I tired for ages with a couple of screwdrivers it may be possible with the engine out of the car but mine was in and everything was in the way.
So i had some large geared circlip pliers that i bent for the right angle of the dangle to do the job.
If you are just trying to move the casing it will move just fine using gentle taps from a hide hammer, it also moves if you stick the hammer handle into the snail hole and lever it around suprisingly easy but that doesnt help for the drilling and tapping, again may be possible with engine out.
Lynall