"drilling the corners of the gear slot and then using a dremel tool with cutting wheel between the drilled holes worked for me.
I used the same process and it worked well.
Factory Lycoming IO390 X, HZ CS 2 Blade,
Garmin Dual G3X 460, GTN 625, A/P 507,
GTR 200 & 20 COMM's, 245 audio, GTX 45R