I installed both the same starter gear and case protecter, I wish I would have took pics for you but i didn't, so all i can do is tell you what I did... first take the covers and gears out, pictures in service manual, put the case protector on first drive it on far enough that the gear doesn't rub on face of case protector then i recommend plugging the spaces to the case with rags or paper towl, somthing like that because when you start removing material for the new starter gear, the little grinding particals get all over the place. I used a little dremel tool with a round grinding stone and used the flat side of it. you can see the area you need to grind by putting the new starter gear in place and you will see it run into or touch the "boss" that holds the little gears between starter gear and starter clutch. you can check if you have taken enough material off by puting every thing in place and see if the starter gear cover(the smaller cover) fitts into place flush with gasket surface, if you haven't ground enough off, the gear will be touching the cover and cover will not fit flush.
As far as modifying the starter holes, I put everthing together including the starter, then checked that starter was ingauging with new starter gear(by removing starter gear cover and leaving gear in place,"AGAIN") and I found that I didn't even have to modify these holes, there was plenty of contact with these gears(more than stock) and its been working great for a year now.
I don't know if this will help,,but you may understand what Im tring to tell you here when you get it apart.
