Sounds like you've found the leak point. Take the opportunity to clean out the cowl — it gets clogged with leaves and trash and no longer drains, and/or the drains get clogged from below with undercoating, paint, road mud, etc. Once the heater box is out you can reach up in there. It helps if you first break your arm in at least five places to allow the needed flexibility!
A Shop-Vac can also be put to good use in cleaning out the cowl, and you'll probably need to get under the car and physically clean the drain(s). You may find you just can't get access to the trash in the cowl from inside. The '63-'66 A-bodies had cowl cleanout plugs left and right. These were carried forward in '67 and beyond in Australia, but not in North America
If it becomes necessary, you can make your own hole(s) in the firewall, clean the cowl, then install an appropriate plug.
....I will clean the cowl area out tomorrow and I do have a shop vac which should help. I found leaves, shells from some kind of nuts and mouse droppings in the heater box. aside from the cracked housing, that I'm going to repair, everything else heater core/motor have no issues. Do you recommend a particular sealer/sealant for the box halves when they get put back together? should turn out pretty nice and the satisfaction of doing it myself with the help from everyone here...thanks, nico