I didn't know any bug eye scorpio had LSD, only traction control system.

Scratch that plan mate...

Over here in the 80's Ford of Finland offered a ZF limited slip diff, this was installed in the regular 7" slot-in differential, and that's pure bolt-on. Don't know if they offered such over there though, so it might be there are no bolt-on options for your Sierra, and the only reasonable way to get a limited-slip-diff is to buy one that fits in the casing. Quaife does one, costs about a grand...
I don't think ZF makes them any more, which is a shame, as they're next to impossible to find these days, and it made for an excellent car. Better than Ford's own viscous unit in my opinion.