Colin,
My car had the original aluminum cross member. I not only wanted to poly-mount my drive train but also move everything forward, so I chose to cut out the cross member and replace it with RCR's updated steel version. I also modified the adapter plate brackets and fabricated the rear mount. To some, it was a big undertaking, but it really was a rather straight forward and easy project.
I believe all new chassis' come with the steel cross member. If I can answer any questions or help in any way, let me know.