[HO455]

The easiest is put a nut and bolt in it.
The right way is to return it and require the supplier to provide the correct part.
Tapping it may or may not a work depending on the size of the hole that isn't tapped. If it is too large then the next size up bolt is likely going to be too big for the bracket.
We Hot Rodders are too quick to modify a new part that is incorrect in order to get things done. But the reality is that after 60 plus years manufacturers still don't know how to make a correct alternator bracket for a SBC since no one ever returns them because it doesn't fit. So they keep selling stuff that doesn't fit correctly.
As I am guilty of this too I will now get off my soapbox.