The only issue I have with doing it that way is that it COMPLETELY changes the rear suspension geometry. Without doing the calculations, there is no way to know if his car will even be drivable.
If want to cram some big tires in the back, I suggest first doing body bushings and carefully centering the frame on the chassis. The second making sure your rear is completely centered. THEN notching.
If you are going to back half the car, I would do it right and go with a complete kit and after market rear suspension.
Im not saying they did it wrong, Im just warning that I dont see any mention of suspension geometry being taken into account and suspension geometry is directly connected with how the car handles.
If I had done this, I would have made sure the LCA mounts were all exactly in their stock position, or just designed a new rear suspension. His method could be done while retaining the stock LCA location, BUT it would be a ton of fabrication.