You have a 3.92:1 ratio diff
You need to keep this ratio
It will normally be in a 7 inch case but you may have a 7.5 inch case as only the UK market had the same thing across the range
Then there are things such as crown wheels and pinions being different sizes between the two, input shafts being different lengths etc
If you were to put a 2wd open dohc diff in it will work but if you are talking cosworth then everything's bigger so wing mix and match