Neither Goldwater nor Johnson will be on the ballot. A comparison of the number of votes they received a half-century ago to the potential votes in 2016 are irrelevant. Neither 2016 nominee will be a Goldwater or Johnson.
What is relevant is who the Parties nominate in 2016 and the issues those nominees run on. Dozens of different scenarios can be speculated on, depending on who the two major participants will be.
There is nobody in the GOP who could have beaten LBJ in 1964 as it was largely a sympathy vote for JFK, whose assassination was less than one year old at the time. LBJ had wrapped himself in the JFK coffin flag extremely well, so the comparison to 1984 is even better. Reagan was running for a second term; JFK was running for a second term.
JFK had really been assassinated less than a year before whereas Reagan's attempted assassination was nearly four years old and had faded from the public mind.
Don't forget also that one of JFK's final acts as president was to sign a massive tax cut which was just kicking in full steam by the fall of 1964.