The 2nd generation AR10 mags are made by CMI and they are excellent.
There is no need to butcher any more USGI M14 mags.
Armalite is selling the second generation of 20 round mags for the AR-10 in what they call a "Five Pack" for $150 which works out to $30/mag. They are also offering a 5 pack of 25 round mags for $200.