Unfortunately your gear case is destroyed. No practical way to repair it. I had the same happen to me twice.
I'm sure you are right, good point.I did notice on the Fiat eper parts program that the diff rear casing is not shown or listed, suggesting that it is sold with a bell housing, probably matched and line bored, similar to cam clamps and heads being matched and line bored.
This suggests that care should be taken to match things up as best as one could.