$3500 in 1964 = $26943 in 2015. But salary increases haven’t (in real terms) kept up with inflation since 1973 or so, hence the longer loan terms.
Don’t leave out the extra performance, convenience and safety features that are standard now. A new car with similar features and performance to a new one in 1964 would be many thousands of dollars cheaper than the average new car price today.
Averages are skewed by the explosion of upscale vehicles, especially SUVs and pickups.