I've done this once, definitely not a fun job. You can leave the heater core in, it' doesn't get in the way. You have to remove the blower motor and the shrouds, the clips that hold the shrouds in place can be a bit of fun to get on/off. I remember the air recirculation flaps get in the way and you have to remove this plastic linkage that's attached to the small motor that closes the flaps. I spent a good hour trying to get them out and just ended up breaking the links off (my car had no AC so I didn't care about recirculation). Taking the wiper motor off is a must as well, I've read of people who have done it without removing the motor but I'd like to actually see someone do it because it seems like it makes the job that much harder. Once you have all that stuff out you just have to manipulate the linkage the right way and it will come out. Another word of advice is to mark the orientation of the blower motor relative to the shackle that holds it in place because it should go back in the same orientation. And taking putting that shackle back on can be a bitch although I didn't have much trouble with it.
And as always installation is the reverse of removal lol.