Absolutely you can sit in the front seat but this will be determined on the day. Pilot's need to distribute weight across the helicopter.
So if the weight cannot be evenly distributed with you as a priority in the front seat, your front seat booking will be cancelled as so you'll have to sit in the back.
To guarantee a front seat if you meet the weight requirements; you must book the front seat if you're given the opportunity well booking.
Not all helicopter tour companies offer such a service, but when they do it will cost you more for what is an upgrade.