The key to achieving what you seek to get, Black Angel, is for the police and heavier forces to be unable or unwilling to respond.
You have the right time frame. The 1970s were not a good time in the USA. The energy crisis, vets returning from Vietnam, drugs becoming a major problem and a great many discontented young people, many of them coming into their twenties and thirties, those who had been in the baby boom. Too young to remember WWII, knowing only the prosperity of 1950-early 1970s America, now facing a major recession, also remembering the hippies of the 1960s, and as such wanting social changes. This is particularly true of African-Americans, who are now getting the same rights as whites.