1999 Buick Regal chaning 99 Buick Regal heater core

Hello
My heater core recently went out. Was driving and car started to smell like coolant. Then passaenger side started to smole and fill up with coolant.
But we cant seem to figure out how to replave the core. The dash is almost impossible to get off and the core is so deep in the car.
Anyone know how to change it for thos specifoc car? Any tips?
Were doing the work ourselves.