As far as the dual cases go, they should hold up well. I would make sure I added a chain or other positive attachment to the engine mounts to help spread the added torque to more than just the rubber mounts. I assume you have a dual mount crossmember under the gearboxes, or the FROR rear crossmember? with the 4cyl or the v-8, that torque has to be transmitted to the frame somewhere, and the additional force from the v-8 will make it more important where and how it's transferred. If you have 4.7's in one of the gearboxes, it will be even more critical. But, the real weak link will still be the birfs. You should be breaking those long before you pull the drivetrain loose or break it.
