to generate 1040-X in Turbotax you can just add "Other income" of $n,nnn where n,nnn is one third of your distribution.
That still leaves you to do the Form 8915-F. see lines 12,15 for a 401k distribution.
Note: this generates incorrect 1040 but you don't send 1040 with 1040-X.
@gmmayes94
