I suppose the answer to this depends largely on what one calls "Existentialism."
There are clear points at which Marx is in agreement with Existentialism, especially a focus on concrete, factical life, especially a life as it is actually lived, rather than as it is determined by some overarching schema. Indeed, the general slogan that "existence precedes essence" sounds not dissimilar to Marx's "inversion" of the Hegelian dialectic. All of which is to say that Marx's views aren't diametrically opposed to Existentialism, broadly understood.
The difficulty is that Marx's own alignment with Existentialism is undercut by what is typically understood as the basic Existentialist premise of the importance of individual subjectivity in contrast to what is the key of Marx's thought, namely the social, or, as he refers to it in his earlier writings, "species-being." For Marx, individual subjectivity is of less importance than its capture by social dynamics, which is to say that he conceives of human beings as first of all beings in relation to particular social forms (compare, for instance, the Kierkegaardian thesis that "the crowd is untruth"). Which is not to say that Marx is inimical to Existentialism, just that the distinctive characteristics of Existentialism are of lesser importance to Marx than they are to more "canonical" 19th Century existentialists such as Kierkegaard, Nietzsche, and Dostoyevsky.
Marx shares with proto-Existentialism influences in the form of Feuerbach, Schelling, and even certain moments within Hegel himself that seem to offer an escape from a crude Idealism in favour of sensuous life and concrete existence. Nevertheless, these influences play out somewhat differently, making it hard to say that Marx himself is in some obvious way an Existentialist.
As to the meaning of life question, you're right that to some extent both Marx and Existentialism urge a need to create or produce that meaning. The question really concerns the sources of this creation: are they the product of the self-assertion of a unique individual or are they born out of a conflict that is best understood as a means of producing new social forms? The answer is not necessarily dichotomous, but the terms in which it is phrased tends to lead to different foci.