42 Ford G 6 cylinder generator

[edhd58]

What am I missing or overthinking here.
This is off my 42 G six cylinder.
I have had several instances where either the generator is moving or the belts loosen greatly, and I am betting on the generator moving after I've tightened it down good. Once this caused overheating to the point of boil over.
When I pulled it off the car the first time it had two same size bolts with a pile of washers over that weird shaped hole. This is also as real pain in the *&^ to get both belts tight and hold everything in place as you're tightening the bolts.
Is there something I am missing, overlooking or is this just a bad design.
Next question is what can I do about the odd shaped hole? I am considering a piece of flat 1/4" thick steel made to cover that odd hole instead of washers.