DE PALMA HELPED TO EDIT MEAN STREETS, WHICH IS TURNING 50 THIS YEAR

In a GQ video posted to YouTube, Martin Scorsese begins at the beginning in talking about how he got involved with Taxi Driver:
Taxi Driver – it really goes back to Brian De Palma, his independent cinema – and Hollywood started saying, "Hey, maybe these indie films, these kids could work in the industry." And so we were all out in L.A., and he introduced me to Paul Schrader. Schrader wrote Taxi Driver. Travis comes from his vision, but more psyche. I connected with it through Dostoyevsky’s Notes From Underground. It was, like, enlightening, you know? And so, for me, when I read the script, Brian gave it to me. He said, You know, I don’t want to do it – I can’t do it, but maybe you should do it. But I didn’t have enough cache, as they say, at that time, to make the picture. But then they saw the rough cut of Mean Streets, and they changed their mind—especially when they saw De Niro in it. And so they said, For the two of you, we could probably get this film made.
Meanwhile, earlier this week, American Film Institute put a spotlight on Scorsese's Mean Streets for that film's 50th anniversary:
Scorsese told a group of AFI Fellows during a Conservatory seminar in 1975 that he encouraged his actors to improvise during rehearsals, which he transcribed from audio tapes to include in the script. As one example, the scene in which Johnny Boy delivers a long monologue to Charlie about why he cannot make that week’s payment to Michael was originally entirely improvised by De Niro and Keitel, according to Scorsese. The sequence was the last to be filmed, after Scorsese pleaded with his producers for another day to be added to the shooting schedule. He mentioned in the seminar that he edited the picture but did not receive onscreen credit due to DGA regulations. Also participating in post-production were filmmaker Brian De Palma and Sandra Weintraub, who Scorsese was romantically involved with at the time. Scorsese told AFI fellows in 1975 that he was working with Mardik Martin on a sequel to MEAN STREETS.
More details regarding the editing of Mean Streets can be found in Les Keyser's 1995 book, Martin Scorsese:
Scorsese admits that he had trouble editing the last sequence of Mean Streets and got some advice from Sid Levin, who receives credit on screen for shaping the whole film. Scorsese maintains, however, that the rest of the film is his work as an editor: "Sid didn't cut it; I cut it. Sid came in and showed me and made an initial cut in the last section where they're singing 'O Marienello' at the end, which is the traditional song that ends all the Italian festivals. . . . At that point, I couldn't cut it. It was five months editing and I was really freaked. The rest of it I cut. Brian De Palma came in and Sandy Weintraub.