Firewall leaks-Help

[1977ChevGMCK2025Fan]

Gents: I have developed some leaks in my 78 K25. On the passenger side, it is leaking into the cab floor through the 2 heater box hose holes; and along the top/side in front of windshield under the black grate. It leaks there around the side into the large vent hole on side of cab. It is coming in under the plastic cover. Anyone had this issue and/or know the fixes? Are there any grommets or something that originally plugged the space around the hoses?
Thanks,
Marvin
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[Wgesnerjr]

for the space around the hoses, there is something under the box that has surely deteriorated by now. I would head down to the auto parts store and get some of the black tar like stuff that goes around the lines on an A/C truck. I don't know the official name of this stuff but it would work great in this application. At least until your ready to pull the heater box and fix it.

[Wgesnerjr]

...this is an old box I have. Numbers may have changed since I bought this.

[truckin 79]

I would say your heater core needs replaced