So looks like I have to replace my heater core

Ok, since this is the guys first attempt at this, I would highly recommend he actually remove the box from the car so he can see the location of all the clips. It's literally just unbolting the blower motor and undoing the J hook. Five minutes of extra work to avoid half a day of poking around and wondering why the damn thing won't come apart...and then there's reinstalling the clips. MUCH MUCH easier with the box out.
I'm doing these cars for 37 years, and when I've got to do a heater core, I just drop the box. Done it both ways, and believe me it's faster and easer with the box out.