Piketty’s paper [1] gives a break out of U.S. tax 1970 vs. 2004. It looks like this proposal would effectively return the U.S. to postwar pre Reagan era norms? If I’m reading the table correctly the effective rate in 1970 for the highest bracket is 74.6%.

[1] https://eml.berkeley.edu/~saez/piketty-saezJEP07taxprog.pdf