I can think of several things that can go wrong when replacing a flywheel - not from the new flywheel, but from the act of removing and replacing the transmission. Fluid loss from the cooler lines or torque convertor hub seal - and not refilling it. Breaking connectors, not installing/tightening roll restrictors or mounts, having the torque convertor unseated from the transmission and bolting it all back together, etc... Good luck!
