Seam sealer question

[x44d80]

This pertains to a 1969 Norwood Camaro, 08 build. The firewall has a beige seam sealer on all seams. SEMS makes a good substitute for that. My question is the black seam sealer that is in most all joints in the cab and rear quarters and the wheel well joints. Its a thick black sealant that reminds me of roofing plastic except that it remains fairly soft but crumbles when removed. Any idea on what would be a good substitute for this black sealer? Thanks in advance.