The data itself tells an entirely different story from the idealized 91% tax rate. According to Internal Revenue Service data, presented below on a graph, from 1966 to 1970 the effective tax rate of an average tax payer in the top 1% was 30.85%. Throughout the time period in question, the effective tax rate of the average top 1% never exceeded 35%.
You will note that is “tax rate,” not “marginal tax rate.” There is income shifting, and also deductions, but also important is the sheer fact of greater income equality. Most income was not in the highest of tax brackets, nor was there — even for most of the relatively wealthy — a plausible way of getting very high up into those brackets. (I thank Balding for some useful discussion over email, without holding him to any particular version of this interpretation.) Here is an early MR post about how sometimes average tax rates matter more than (reported) marginal rates.
For later periods of time, here are some good tax visuals from the NYT.