How far would a barrel have to be set back to do this rechamber?

Pretty much the length of the case. Most barrels won't have enough shank to do a set back of that much.

Depends on the AI chamber. A minimum AI chamber will only be .005" larger in diameter at the shoulder than a minimum SAAMI chamber. If it was set back .130", you would end up with a biggish standard. If you set it back a half inch, it would probably clean up to a SAAMI minimum. GD