Heater core replacement procedure varies widely between Beaver coaches. On the original production coaches (pre 1995), the core was removable through the removable front panel in the front cap, and most mechanics remove the entire evaporator case to replace the heater core. On the SMC era coaches (1996 to 2002) with a removable panel in the front cap, the procedure is like the original design, but if there is no removable panel, you have to remove the top of the dash, and cut an access hole (the factory access hole is usually too small) to remove the evaporator case from there and from under the coach but most of the work is done from inside the coach through the top of the dash. You have to be careful when cutting the access hole and repair it when finished (I reinstall the piece I cut out with a sheet metal piece above it) to prevent air leaks, but it is doable for the average mechanic. The same basic procedure is used for A/C expansion valve replacement, but the evaporator case is not removed.
Gerald