1965 273 Intake Manifold Gaskets

Welcome to the site. Look around for Mahle MS15125 or Victor 111053101 as well.
Rockauto shows those PN's, but both "Out of Stock". You can click their "Notify Me When Available Button", but can be a long or even infinite wait.
I wonder if a later year could work, even for the Magnum engines. Might need to enlarge around the bolt holes w/ gasket punch, and some later heads had taller intake ports, but might still seal.