Faxon's book of 1967-9 A-body assembly drawings may be something you want to buy for what you are doing.
There's a review of it posted here so I won't repeat that.
What I did for some drawings is photograph or copy and then mark up so they would be easier to work with. One of those happens to be the drawing of firewall pass throughs.
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and some the grommet drawings that go with it.
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