Its an aluminum block.

I'm sure there are many reasons, one appears to be it is just a better option than the production blocks at this power level. Weight distribution I am sure helps his cause as well. Especially at the weight they run and limited tire. He has also changed classes which I am sure is a result of the aluminum block in part. The factory showdown FS/AA version of his car has to weigh in at 3550lbs while in FS/A where he has the car now he can be down to 3410lbs, but he is says he is still at 3500lbsas he runs the car now. So he still has more weight he can remove from the car.