You'd need the rear suspension from a 2wd cosworth, as scorpio diff is 7.5".
Normal Sierra diff apart from 2wd cossie is 7", so scorpio diff won't fit in the sierra rear subframe.
Also, Scorpio diffs have flanges for driveshaft attachment, but only Sierra's with 4x4 or ABS have flanges, all others the driveshafts just slide into the differential.
Then you'd also need a bigger master cylinder(or brake booster from ABS equipped Sierra/Scorpio), as the small cylinder is only suited for disc/drum set-up.

So...it will fit, but it's not exactly simple.
If your car has ABS though, any differential from a 4x4 Sierra fits bolt-on, and at least over here all 4x4 Sierra's had a rear LSD.