1975 (and earlier) hvac distribution duct (heat and AC)

In 73/74, the only way to get air out of the AC box was via a slot in the top of the box that opens into plastic ductwork on the inside of the coach at the top of the firewall.  This duct opened into other ducts both inside and outside the firewall.   It\'s often very poorly sealed and leaky, and is a pretty long path with a lot of bends for the air to go through.   In 75/76, they added a kind of short circuit in the form of the "max air" slot.   It uses the left 12" or so of the existing recirc air intake at the bottom of the dash to blow conditioned air, and added a vacuum controlled door so you could still use the existing ductwork at the top of the dash.   Unfortunately, they used the vacuum pot from the fresh air inlet for this, and sealed that door off.  This meant they reduced the air intake for the blower motor to well under a quarter of its prior size, which reduces the capacity of the blower.   I enlarged the recirc/intake slot as others have suggested, and this certainly helped airflow, but it\'s still not a great system.   I think there are better solutions out there...