There are lessons to be learnt all the time working on cars.
One big tip is bunnings don't know or care about bolts. Go to a proper bolt shop for correct grade bolts and advice .
How rooted were the threads? I know that it's too late now, but a die nut would have likely cleaned them up. (Or a thread file, which works out cheaper and more flexible)
Hope you start having some better experiences in the future.