Unless I am greatly mistaken, the M1A barrel is not threaded. Maybe not even the version of which you speak.
As to your second question, I.E., a permanent attachment, perhaps thet might interfere with the removal of the gas piston. If so, that would be a sure-fire NO-GO.
Suggest you contact Smith, et.al., with rifle in hand and measurement tools, for an exact answer.
Good move on the Vortex, though. My experience with shorter-length 7.62mm barreled rifles leads me to the conclusion that they are dead give-aways in dark conditions unless an effective FS is installed.
Slightly O.T.: There are probably thousands of firearms owners who have never taken the trouble to fire their guns (pistol, shotgun, rifle, whatever) at night. My guess is that a good many would be horrified to learn that, due to muzzle flash, they will have only one aimed shot.