Joe, I had a 2 1/2 hole saw. Put it in a drill, spun it in front of the grinder and ground everyone other tooth out that was sticking out. That fit perfectly in the factory opening that I drilled out without the 1/4 inch drill installed. That worked to remove the back plate.
Then I took another 2 1/2 inch hole saw and enlarged the opening a little. Not enough. So I used a half round file and went to work. 4 hours later the starter fits perfectly.
There isn't enough money for me to do this to another GX390, NEVER AGAIN.
I also had to drill the flywheel to tap the 3 places for a flywheel puller and had to tap the place to mound the charging coil. Once is enough for me. Wish I had removed the metal filings from drill press. They stuck to all 3 magnets on flywheel, gray moment.
There were no instructions included, so I called deezee and they emailed me the wiring diagram.