Sierra Diff info

Are there any Ford experts out there who knows the origins of the crownwheel/pinion set used in the normal Sierra 7" diff ?

I like to change ratio from 3.9 to something like 4.4 but without shelling-out £500 for a motorsport cwp set!  Working on the theory that Henry would be more likely to use components already available rather than making them from scratch. Have looked at internals of single-wheel Transit diff but pinion shaft is shorter.

 

Anyone explored this area before (or know someone who has) ?

Any advice/pointers greatly appreciated.

 

Keith.