You are just dull. The top rate applied only to the money that fell over the previous bracket, so 91% was only on that last chunk of money... You are talking out both sides of your mouth trying to ignore that Krugman either doesn't understand the difference or hopes saps that still read him don't get catch it. "Gee, people used to pay 91% of their earnings to the government, derrrr...".
Also, Krugman completely glosses over the fact that there were 3 recessions in this great period he describes, thanks Kudlow for pointing that out. Economy picked up under Kennedy, who cut taxes. The substantial recessions. Most people think fondly of the 50s for the family values; Krugman must just like financial downturns. He will be very happy for a while.