Not OP but he's right, your statement is slightly misleading, not many people actually paid that (fewer than 10,000 households[1]). A good source as you asked for can be found here:

[1] https://taxfoundation.org/taxes-rich-1950-not-high/

The Tax Foundation has been called a “deliberate fraud” by Nobel laureate Paul Krugman. (https://en.m.wikipedia.org/wiki/Tax_Foundation)

Also, 10,000 households in the 1950s is roughly equivalent to the top 1% of the top 1%, and exactly who i’d expect to be taxed at a rate of 90%+.

I'm not saying the 90% number did not exist. My point is that overall government was lower then. What part of that are you not understanding?