Can you use the old crossmember? I thought the 4 speeds and the zf were very close to being the same. When I swapped my zf in where a c6 resided, I found the lower crossmember for the c6 was exactly the same as the zf lower crossmember from the donor. It just needed to be slid back 1 inch. The short upper crossmembers were different, the zf short uppers had more offset to compensate for the 1 inch slide back, and still used the same frame holes.