You need to undo THREE bolts that join the FLEX PLATE to the TORQUE CONVERTER. They will face the engine. Undo them with a large breaker bar, and use a breaker bar on the crank pulley bolt against the chasis rail to stop the motor turning. Alternatively, jab a cold chisel in against the side of the block through the flex plate to stop it turning.
Once the torque converter and the flex plate are separated, you can slide the trans back, and lower it down.