Sealer between upper and lower cowls on 65 Dodge Dart

I’m getting ready to weld the upper and lower cowls together, and to the firewall on my 65 Dart. There appears to have been some kind of black goo between these parts from the factory. Does anyone know what I should use to replace it, or if I even need do it? I will put seam sealer back along this joint and will also seal along the top of the firewall inside. Photo of the area is attached.

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