AOD won't clear the early X-members,whereas the C4 will.I have a C4 with a Gear Venders OD in my 40 with no major X-member modifications.Now I think this same kit will also allow the use of the AOD which has a lockup converter and an OD. Some think it needs more power to operate, but a friend has one and it works great, burnouts and gas mileage, can't beat that with a stick.