Apache,
If you look at a Galil and the standard Galil stock, it has a circular fitting on the end. Kind of like the end of a pipe. If you look at the rear of the Galil receiver, you'll see a roll pin. The pipe end of the stock fits into a socket in rear of the receiver and then the whole shebang is pinned by the roll pin.
The simple answer for a milled receiver would be to remove all of the stock tangs and mill yourself a square block with a big, round hole in it. I actually think a good metal epoxy would hold it in place. Insert the Galil stock and then drill a hole right through from one side to the other and pin it.
If you take a look at the force vectors, the strength of the steel block/trunnion keeps the stock extension (pipe) in place and if it is a tight fit, won't allow side to side or up and down stresses create any movement. Solid. The pin simply keeps everthing in place in the receiver. It would only appear to be suspectable to any shearing forces directly down the axis of the weapon, i.e. a very hard blow directly on the butt. Even then, it would have to be huge to shear a large roll pin.