When I added mine, I mocked up the sway bar to get the position of the tabs where I wanted them. Then I had to grind a notch in them to mate against the ridge. Once this was done, then I just welded them on.
Since your arms are off and you want to set them up first, You might mount the sway bar, align it to the current arm and mark where you want the tabs, then transfer those marks to the replacement arm.