2WD Cosworth diffs, are completely different to lesser models, also as you found out, as is the rear beam. Lesser models were 7" diffs, 2WD Cosworth are 7.5".

The rear suspension setup on any model derived from the Granada, but I believe these have have 7.5" diffs (Probably the same diff, if so one of these would work?).

Also all Cosworth varients have a LSD, most other models don't, so you don't want to be fitting the diff you have!

As said though ultimatly it's bushes that let these down due to age (What else can you expect from perished old rubber)

A poly diff mount for £100 is a good investment, and maybe check the condition of the beam bushes, in particular the beam mount bushes.

Martin