To replace the heater core per the FSM directions requires the freon evacuation and removal of the AC evaporator and HVAC box. Not to mention the entire dash... Being that this is a ton of work and involvement of an AC tech, there was many many threads and discussions about an alternate method which reduced the process to rocking the dash back enough to reach the core. Then cut the lines, remove the core, and cut the new core lines to match. Then install the new core and reconnect the lines back together with clamps.
I myself chose this route and did a clean install. Worked perfect... If you're interested I can link the write ups on how its done.