Heater Core??

If I remember correctly the heater box is in 2 sections with a seal gasket between them. If I remember right the gasket was just compressed foam. The seam has a groove fit and is held together with about 10 strong “c” type spring clamps around the perimeter that can be pried off with a screw driver allowing the box half’s to separate. Some of them were difficult to reach under the dash but doable. The harder part for me was getting them back on. Be careful not to lose them. I also removed my fan and diverter door for rehab while I was in there.
I originally ordered a replacement core from Summit but had like a six week back order so I cancelled it and had one from NAPA the next day. I’d guess you can probably get one from Rock Auto too.
Good luck.