Remembering that the Belly of the Whale is a very general term, in Goodfellas (1990) it is:
Henry meeting and dating Karen, with whom he'll embark on the deeper change.
It's Henry going on a cruise with the stolen money, against Paul Cicero's wishes, which will lead to the eventual detachment.
The 2000+ stage Hero's Journey and Transformation through a New World/State is the template upon which the vast majority of successful stories and Hollywood blockbusters are based upon. Learn about this at http://www.clickok.co.uk/index4.html