Modifying Actions, Safely

[tiriaq]

Probably the easiest way to make the cut off slot would be with a slitting saw in a vertical mill. Ends of the cut would need to be finished. Then the hole through the boss would need to be drilled, tapped and countersunk.
Cutting this slot would do nothing to increase the strength of the receiver. Personally, I would only use somewhat reduced handloads in 7.62x51 cases.