I have just replaced the heater core in a C34,with one from an R33, the existing core was maked as Laurel. The core had a hole in it I think due to no green stuff in the water.
It was leaking into the drivers footwell through the ducting, with less water in the passenger footwell.
As it was the first time I have had to do this, I removed the entire dash pad, centre console, loosened the steering column and everthing to get to it, after doing it I think I would be able remove the core without taking the entire dash out.
The hassel was removing the passenger air bag to get at the fan out of the way to access the core. It is a mission even for me who takes cars to bits on a regular basis.
There three units that make up the heater setup, Ducting director, fan assembly and heater core assembly.
Took a day to get out, replace and back in.