No - the 2 Jimmy Conways are completely different people. In Goodfellas - the real Jimmy Conway was actually called Jimmy Burke. He was alive at the time of the Goodfellas shoot and his name had to be changed - see below.

That is just a coincidence.

Once Upon A Time in America is is partly based on a book by an ex hood called Harry Grey. His real name was David Aaranson and he composed it in 1952 while in Sing-Song Prison. Apparently, it is said to be partly autobiographical - but I don't know which bits. I am sure one of the experts know.

But it was largely conceived by director Sergio Leone - who saw it as an unhappy fairy tale for adults - hence the "Once Upon a Time" theme.

There are some old posts about it. If you look in the General Discussion section there is a thread
about Once Upon A Time America Movie - last post
on July 9th. I don't think it deals with the truth issue - but some of meaning of the movie is explained.

There are also good posts about the origins of Goodfellas. Refer to Turnball's answer to Corey Bova's question about Wiseguy = Goodfellas posted accidentally in the Other Puzo Novel section. Based on a true story, based on Henry Hill's experiences - some of the names and details have been changed. The film was aided by the book "Wiseguys" by journalist Nicholas Pileggi in coordination with Hill. Pileggi was also responsible for the book surrounding Casino's true life main protagonist Lefty Rosenthal.