Whether or not you believe the "ghost stories," this house hosted the murder of 6 members of the De Feo family by son Butch. The house is centered, is on the west bank of the canal, and is oriented west-east just by the "L" jut of the bank.
The actual house on Long Island was not used in the movie. Renovations were made to a house in Toms River NJ and that house wad used as the Amityville house
http://www.newsday.com/classifieds/real-estate/amityville-horror-house-back-on-market-for-1-15m-1.1945998
http://virtualglobetrotting.com/map/the-amityville-horror-1979-house/