The original heater box internal door flaps ('70 Cutlass non A/C) had material (foam?) applied to the surfaces to create a good seal and insulate from noise of the flaps opening and closing. The material is deteriorated and disintegrated after 51 years. I want to install new foam seals on these internal heater box door flaps. Anyone have experience opening the heater box to replace these seals? I looked for an existing thread but didn't find one. The heater box cover appears to be spot welded. What is the best way to remove the cover to open the heater box so I can access the internal door flaps? Thanks in advance.