Heater core gave up on me.

[2many projects]

I'm new to the board and own a '71 Chevelle SS 454.
I've owned this car for 20 years and it's very well kept and the pride of our small fleet of rods and other specialty cars.
The heater core finally died on me last fall and it's getting near time to change it.
The Chevelle is a factory big block, automatic and factory installed air conditioning.
My question is ........... With A/C, does the heater core come out from underneath the dashboard, or do you pull the heater core off the firewall under the hood and rt. fender area?
My shop manual was useless and other Chevelle owners I know all own small blocks with no air.