When the film wrapped production, it took over a year to edit and score because the producers needed to secure additional financing.
The title on screen is actually a shot of the back of the jacket of one of the gang members, and the ungrammatical "The Lord's of Flatbush" is exactly how it is rendered, to show that these guys may be lords of Flatbush, but they're not lords of English grammar.
Richard Gere was originally cast as Chico. According to Sylvester Stallone, he and Gere fought constantly including physically and he demanded Gere to be fired.
Chico's real name is David.
When Sylvester Stallone was making Rocky(1976), the producers showed the studio this film so they could see who Stallone was. The studio mistook Perry King for Stallone and was excited about him playing Rocky. When the producers pointed out that the guy playing Stanley was really Sylvester Stallone the studio's excitement faded.