It may help a bit to disconnect the heater core.........but.       
What I found was that a lot of hot air comes in through the heater even
though the heater is cold,  when it is very hot outside .
This is because the foam in the heater box flaps disintegrates with age and
allows a lot of air past, even in their closed position.
I have repaired mine, based on an old posting of Karl Wolfe I found in the
archives and now  I dont get hot feet anymore.
The a/c has more of a sporting chance to work also.
Unfortunately it requires taking off the dash and removing/dismantling the
heater box.
Not a terrible job though.

Steve Cotsford
'84 Westy