Mounting to an old pan-style electric box
How annoying!  OK get a set of drills and start sticking drill bits up through those holes -- butt end first.  We just need to know the hole size.
Now hit a &quot;Drill and tap&quot; table.  Anytime you tap a thread into a hole, you drill the hole to the smaller size (tips of the threads).  Go to the table under &quot;Fine thread bolts&quot; and find the fine-thread size whose drill size is slightly bigger than the holes you have.   Buy the correct drill and tap, drill, and tap.
Now you have what you thought you had in the first place.
You need to use fine-thread because coarse-thread won't have enough purchase.