Well there's this which goes back to 1967: https://www.census.gov/library/visualizations/2025/demo/p60-... I'm having trouble finding the raw data though. In any case I'm not seeing any big drop in the 1970s.

That's household income. You need to adjust for the change in households with multiple earners. That's why I said the census data is dirty and conflates things. The number of households with both parents working increased from 46% to 52%, so median household income staying flat means median income for individuals went down pretty significantly.