^^^This^^^I would do some DIY alignment before I slotted the brackets... Maybe they won't need it, but if they do you will know which direction to start cutting.
Adjustable arms seem like a more solid way of aligning a straight axle, I would assume a slotted bracket would be far more vulnerable to getting bumped out of whack
Plus a DIY alignment is just as good as one at a shop. They can't do anything you can't in your garage with a tape measure and some patience. Maybe someone could post up Eddie's write up for the DIY alignment.