I had to replace a heater core in my 68 Sport Satellitte way back in the day. It was an AC car. Like roadrunnerh said above. It can be done without removing the whole dash frame, just the stuff in the way....Once you remove the glovebox, radio, dash vent tubes, etc. it becomes easy to remove.
Now a days seat removal is a must. Dang I'm old...console shift. I had planned on removing that and seats.