All aftermarket blocks both iron and aluminum use a billet steel cap. The aluminum cap deal came along to try to keep production blocks alive in big power applications. The thinking is that they help absorb the "shock" that tends to crack mainwebs and caps. The real problem is usually from detonation but that is the thinking. Does it work? Well if you look at all aftermarket blocks (not just mopars) they nearly all have a steel cap.
Todd