Motor mount should have nothing to do with the starter. Maybe try a 1/2" or 9/16" drill to start with? Just a thought....

 

last year we had a few chevy trucks come in due to the outside bolt hole on the block having a slight crack causing the starter to crack or just make all sorts of noise. we ended up making a bracket for the back side of the starter all ghetto with angle iron and bolted it to the block and that fixed the problem... but who knows for how long

 

I've seen/had a few trucks with a factory support bracket from the starter motor up to the engine block.

 

trust me if you own a diesel 6.2 ect and you dont have the bracket installed by the factory at the rear of the starter it eats starter bolts like candy, and it dont matter the grade of bolt - Dont ask me how i know