What are the design Airflow and Pressure specs for the fan you are after? 300-400HP seems like a vast amount of drive power for one fan. Try talking to Elta
don't know much about what you are trying to do but if this is for air ventilation in a tunneling project would it not be better to have two smaller fans instead of just one large fan in case one of the fans failed there would still be some air cirrculation.