" the question is HOW did they reduce their income so as not to pay the tax?"

Higher Allowances for mortgage interest deductions, personal loan interest deduction, Gold plated pensions, health plans, allowances for General Partnerships, Limited Partnerships,...

Through to the 1950, US Gross Private investment averaged around 16% of GDP, Prior to the GFC it was around 18% of GDP, so investment was lower in the 1950's than today,  not triple.

https://fred.stlouisfed.org/graph/?id=A006RE1A156NBEA,