So I ended up taking everything out.
Here it is in progress.
My hands were not nimble enough to be as surgical as coronan.
I ended up pulling all of the duct work while I was in there. It all seems straightforward enough to reassemble (after it all gets cleaned up) and the new heater core fits nicely.
I do have one hurdle to clear before reassembly... the linkage on the left (driver's) side of the heater core housing (see last photo in coronon's write-up) has one spring that seems to somehow connect to what he refers to as the "vent select linkage". This spring is broken and I'm not sure where one end of it attaches. The exploded diagram I viewed was not detailed enough to show the linkage assembly or spring.
Any body have an idea where this spring goes or how to get a replacement?
Cheers,
Daniel