I am guessing that 1:20 is a typo for 1:29 If so that should do for up to about 350 grains bullets. I measured a couple of .457 cast bullets; 310 grains is roughly .8" long implying that 350 grains would be about .9" long. A 450 grains bullet was 1.25" long
That translates into twists of 1:36 and 1:32 for the light bullets allowing 1" faster twist than the formula. The heavy bullet would require 1:23 again increasing twist by 1" from the formula.

cheers Doug