Joey Zasa is based most on Joe Colombo. Remember that he had his "Meucci association" which was a clear reference to the Italian American Civil Rights League of Joe Colombo.

Some newspaper article I once read about him stated that he was also based on Joe Gallo.

And he's probably also based on Gotti who was the most famous gangster at the time Part III was made. Coppola and Puzo were most definitely aware of that and that may just be the reason they created someone like Zasa.