Look at the blend door actuator replacement instructions. It's directly underneath the blend door actuator. Once ithe BDA is removed, it's like 4 more small bolts, but you also need to pull the dash back further than if you were only doing the BDA.
The first time I did the heater core in a Mark VIII, I did the BDA at the same time, and it only took me 4 hours. The second time was about the same, but I had to stop half way and get a different heater core. The one I had was suspect, and didn't look like it was going to hold up very long.
The hardest part is the spring clamps on the heater hoses going into and out of the heater core. The prongs that you need to squeeze to loosen them are pointed downward, and very hard to get to. I use a set of 45-degree 12"-long needle nose pliers, but it still sucks.