So looks like I have to replace my heater core

You never have to actually remove the dash to do any A Body heater core. Drop the three bolts on the column, and pull that down. Pull the two bolts that secure the bottom of the dash frame behind the kick panels. Then, just pull the bottom of the frame outwards and you've got TONS of room to do whatever needs to be done.
Flashback....I spent my whole youth working on these things outside in NYC winters. Made me the man I am today!