This is a subject that M1A owners that are interested in getting the most accuracy potential should understand. I say should, because if it is not understood, how would you know when you have it. Maybe that sentence should read, nobody likes to admit they don't understand in public.
The upper assembly needs to be at a right angle to the platform {stock] in two places. The receiver needs to be 90 degrees to the stock routing. The most practical place on the stock to make that determination is across the heel area where the receiver heel is going to ride on the stock. The second place is across the top surfaces of the stock ferrule tenon.
You need a quality level and means to hold the stock secure to make this determination.
Most of the time you are going to find, even with the Top of the line stocks, these two horizontal surfaces are not in compliance. This is not the time to start removing stock material however. There is a third factor, the obdurate stock ferrule.. It is not going to make any difference rather your ferrule is standard, as issued, or modified, opened up for clearance, at this point.. A modified ferrule does not mean it is going to locate in the proper position.. Please read that last sentence again.
With the stock secured in a position where the heel surface is level, hand install the ferrule, may need a tap or two, now check across the top of the ferrule. Sometimes this brings the desired compliance into being without stock tenon modifications.
If, with the ferrule installed a non compliance condition still exists the modification should be done to the tenon.. If you attempt to make mods elsewhere, chances are the stock will become Rojkohed.. Art