I was in the neighborhood.
Mary Jane comes out of an audition and runs into to Peter. The two talk and MJ invites Peter to join her and Harry for dinner. Peter declines, and MJ walks off into the rain, where she is accosted by some thugs in an alley. Luckily Spider-Man saves her.
This entire sequence, from MJ leaving the audition, her entering the alley, the fight in the rain, and the iconic kiss, all take place on the Hennesey St. portion of the Warner Brothers backlot. Mary Jane and Peter talk on the part of the street shown below, with MJ walking off around the corner.
MJ enters the “alley” and is attacked by the thugs at the location below, with the kiss happening in the wedge between these two buildings.