Water leak inside 65 barracuda

I believe there is a lip and a trough around the air vent openings in the cowl.
It is a favorite place for debris and rust to form causing leeks.

If you pour some kind of sealing agent though the cowl openings on the hood this can actually make the leaks worse.

Your best bet is to remove your ducts from under the dash and repair whatever damage you see and re-seal it.

Good luck.