Sounds as though you personally disapprove of their proposal, and I believe you should disapprove professionally as well. Maybe if they can't provide a common internal trip mechanism, they could provide interlocked shunt trip breakers. When they investigate the price of this they may be able to find a way to custom build mounting hardware for a 400 amp breaker. After all it is a custom built panel. Or tap the main buss for a 400 amp switch mounted beside the board if you approve.
