I'm in the process of trying 20MHz with the AtMega328P. Every tutorial start with the modification of the board.txt file which I correctly found on my mac under the arduino package Arduino/Contents/Java/hardware/arduino/avr/boards.txt
My problem is that my modification doesn't appears at all in the IDE. I even tried to delete the file and the IDE still show me all the boards... I should do something bad obviously.
I expect there is some other hidden places that stores boards information but I cannot manage to find it.
If you've updated your Arduino AVR Boards version then it's located at:
/Users/{username}/Library/Arduino15/packages/arduino/hardware/avr/{version}/boards.txt
I second using MiniCore but it can be educational to open up boards.txt and have a look.