1965 Other Pontiac Models Trying to remove the heater box/fi

I've gutted my interior (GTO) and I need to remove and replace the firewall pad.
It's behind the heater box, (if that's what it's called)
It's the large black box that connects to the defrost ducts and the floor heater.
It looks like I have to take the front passenger fender off to get to the bolts to remove it.
Can you tell me the best way to remove this box?
Thanks,
Paul