Unfortunately, the terms anti-squat and IRS don't go together very well.
You can increase anti-squat by angling the front mount of the lower arm upward from the rear. This will raise your instant center (the imaginary intersect of the angle of the two control arms in side view) and in turn your anti-squat figure. However, by the time you've gained any appreciable anti-squat you will have reduced your side view swing arm length (distance from rear axle to instant center horizontally) to the point that you will experience brake hop. The solution is then to instead just add weight over the rear (like the rear mount transmission on the C5 'Vette).
Otherwise, if you're already going to go through all of the fabrication trouble, you will probably be better off with a solid axle system. It is easier to fabricate, typically will handle more abuse, and can be designed to give greater than 100% anti-squat.