Maybe you got some of the bolts mixed up? It broke because it was too long, and this new one won't go in because it, too, is too long?

So there's 3 bolts for attaching that mount to the transmission, are they all the same?

 

I ask again, what is wrong about walking into a GM parts department?
I bet it didn't go in because it is not metric or the wrong pitch if it is. SAE and metric threads are not interchangeable. Many US hardware stores don't carry metric .

 

Around here they all have them. At least the common ones. They would have that bolt he needs.

 

Greg you’re in MI the OP is in Florida. Possibly different inventory