I need a limited slip diff to suit a 7" sierra
casing.

depending on how much you want to spend.
tran-X should help you if you areready to pay £600 for it

click

quite a few here, don't know if there's anything suitable?
ntdwm btw

Some Cosworths and all of the 4x4 Sierras have 7"LSD's as standard 2.0L DOHC 4x4 has the 3.9 all the rest have the 3.6 and they are sooo much cheaper than a Quaife or Trans X
I do however like the sound of the Quaife ATB