As LCGuy relates, in addition to insertion of the replacememt ROM chips:

HI (342-0639) at C13

MED HI (342-0640) at C12

MED LO (342-0641) at E13

LO (342-0642) at E12

the IWM chip must be replaced with a SWIM, but you also need System 6.0.2 or higher lest the Macintosh II still mistake the high-density drive as an 800kB mechanism.

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