The ducting he took out is likely just for the a/c to travel from the center of the car to the vent ball in the panel. The heater is behind that much more substantial black box bolted to the firewall where the air comes into the car.
Take your time. You can do it. I also had two cracks in mine and bit the bullet and replaced that very expensive ($300+) piece.
The more I think about it the more I think you won't have to take out the driver's side pad. If that is the case, I would definitely do it myself. If not, I would consider taking it in.
Here is a quote I plagarized from a site that will go unmnetioned.
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I just finished doing my heater core on a 77 C3 for the third time, so I think I know what I am talking about. The last time took me all weekend but I work very slow and changed out my console as well. Here is what I have learned (at least on a 77). One: You do not have to mess with the driver side of the dash -- you can do the job by only removing the gauge cluster and passenger side of the dash; however, there is a trick to not having to mess with the driver's side. The trick is that once you disconnect the hoses, loosened the plenum from the firewall and removed the shutoff valve and cable on top of the plenum, you must slowly TURN the plenum so that the side facing the firewall is on top. This turning action will put some stress on the remaining duct work that you left installed behind the instrument cluster but it can be removed and installed this way. If when you try this, you simply do not feel comfortable with the stresses you are putting on the remaining duct work, then you may decide to remove the duct work behind the gauge cluster but that may involve messing with the driver side of the dash. Once you remove the heater core, you can figure the rest out for yourself, BUT here is the MOST IMPORTANT TIP: connect the heater core hoses to the new core after you have installed it in the cradle and test it while it is sitting on your garage floor (engine running) to make sure it is not leaking before reinstalling the heater core into the dash. The reason is because the tubes on the heater core do not fit very nicely in the cradle and if you force them, the tubes will leak where they are joined at the top of the core. To ease the tubes in the cradle, only loosely attach the cradle bands at the top and bottom of the core. Do not tighten the bands down until you have slowly screwed the clamp between the tubes down. If you tighten the bands first, you risk putting too much stress on the tubes where they enter the core. Once you have tested the core outside the car, you can install it, but I recommend that you buy the heater core gasket kit. Its about $40, but the kit has all the gaskets needed for the duct work joints. Good luck and please let us know what you learn as you boldly go forth.
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I was looking for another post that I had seen recently about checking the air intake area for the HVAC system. It is in that area and as I recall, they recommended taking a look at this as well while you had the heater core out to make sure that it works, basically closing the flap on max air and opening it on regular air and that the flap, which is plastic, is solid and does not deflect excessively. Evidently if this does not funtion properly it will introduce hot air into the HVAC system and is one of the problems with hot air in the cockpity. I'll take another look and see if I can find it for you these evening.
Bob
