Glad you got it sorted out.
The air dampers have rubber seals along the perimeter which tend to rot away and let air flow by - particularly to the floor when the dash vents are selected. Once you pull the dash, they’re easy enough to replace and the GM parts are still available. It helped some in mine, but airflow is still pretty poor compared to anything new today. Even the generation after ours was a vast improvement in the blower department.
By design, some air is sent to the windshield and side defrost vents at all times. Seems like a waste of air when every little bit counts, but that’s the way they did it.
The best solution that I’ve found is to close the two passenger vents when I’m driving. If your heater hose valve isn’t working, that’s worth fixing. It closes coolant flow to the heater core when the temperature dial is in the “max” detent and can lower vent temperatures by a degree or two.