67 A/C heater box rebuild and install help

From the outside it looks like any other hole drilled in the case. Those 2 samll ribs inside help hold the J bolt upright and reinforce the area a bit too. So the hole would be toward the back of what you have broken out there. The kep washer nut on the J bolt is about 3/4 diameter so it will hide most of your repair. That special J bolt has a perscribed thread length to prevent over tightening. We can guess the break came from pulling the case away from the firwall before dettaching that hook.
Adding a larger washer will work but it should be very thin. Otherwise tightening could break out a larger area. Hope this helps