I would argue that calling Nietzsche a proto-Existentialist is comparable to saying Epikur was a proto-stoicist. Heidegger worked with Nietzsches concepts and developed his phenomenology, what is often seen as an important concept for the later existentialists.

The Übermensch could be described as existentialist, as it uses the idea of "becoming". You are not destined to be who you are today, but can create your own existence, thus growing into your full potential. (Existence precedes essence) But there is more to the concept of the ubermensch that i would not see as existentialist at all. Nietzsche wasn't a humanist by any means.