Mainly, where are the drain holes? I've got a 75 Ranchero, but after running the AC a while, I start to get water dripping on my feet. This screams to me that I need to clean out the drain holes for the AC box, but where are they?
My guess, maybe the case itself has a leak...insteak of it draining out in the engine compartment?
Worse case, take the dash out, and replace the seals going from the box to the firewall, and check the ac box itself for signs of water leaking.