The scope mount is supposed to butt up against the step at the back of the top strap to act as a recoil lug. This takes all the shear load off the mounting screws so they don't shear off due to recoil.

If you need to move the hole pattern back, put an equal spacer between the mount and frame to retain this feature. Screws are a clamping element, they can't take much of a side load. If the screws shear off, the scope and mount will come off the gun, probably hitting you in the face.

As scooter123 says, it is important to do this job correctly. You only get one chance to do it right.
I always use a spring loaded tap guide in the drill chuck when tapping to insure that the tap handle stays perfectly straight so I don't put a side load on the tap. Side loading the tap is the #1 cause of breaking the small ones.