65 Barracuda Front end Rebuild several questions and a couple of observations

You would find that dense foam gasket material in a bunch of places where sheet metal meets cast metal. Master cylinder to firewall, bumper brackets to body, etc... It fills small voids, wicks moisture out of those to helps prevent rust. The same material is commonly used in packaging so that's where I get new to cut gaskets from.
I would install new seals and not worry about the missing grease trap. That trap was deleted in later models anyway, suggesting it never was req'd.