After I returned from my dinner engagement last night I headed out to the shop. Whereupon I promptly destroyed my last sanding/grinding drum. So I had Amazon overnight these replacements at $7.99 for 210 drums. (And nighttime is over, so where are my drums?!)
https://www.amazon.com/dp/B0D2DF2RGK
A drum sander/grinder is all you need to grind the notch. Aluminum burr bits are massive overkill; they don't provide the fine control you need. It's like using a bulldozer when you really need a shovel. Intended for Dremels, the drums have 1/8" shanks, but my Ryobi right-angle grinder can accommodate them. The drums' 80-grit aluminum oxide cuts through the crankcase's aluminum with no problem. Forget drills, forget hacksaws, and especially forget aluminum burr bits — drums are the answer.