Your diff is a 7.5" as fitted to the Granada 100mm joints, and Sierra Cosworth 109mm joints. You need a 7" Sierra unit to fit a 4:1 cw+p, Kam Diffs make them.
You'll be lucky to get much help on this site, it's full of kids and dreamers telling each other stories. If you're over 12yrs you'd be better served on Locostbuilder.
If you were in UK I'd buy the 7.5 off you