One word on the use of FAL magazines.....the FAL receiver acts as feed lips of a sort, and the FAL magazine was designed around that.
You might be better off maybe trying G3 or M14 magazines.
I know that Armalite looked hard at the FAL magazine when they brought out their AR10 years ago, but decided on the M14 style magazine because of issues.
Bushmaster tried a .308 AR rifle that used FLA magazines and they also had issues......
Good luck in your project!