We used an AM14U with the 8wd upgrade and Vex Pro 3 CIM ball shifters. We also added a bellypan of perforated polycarbonate similar to
this. It significantly stiffened our frame, was easy to bolt on, and was used as an electronics board. We also added some aluminum gussets and brackets to help add strength.
We beat our robot hard this weekend and had no issues with the drivetrain other than some field debris in one of the belts. I've been very impressed.