What height should i mount the sierra diff? Bearing in mind that the diff i have got is the 9 inch (didnt have to pay for it, thats why i got it, i know its heavy, dropped it on my toe!) Measurements to the centre of the flange would be handy.

I have neither seen nor heard of a 9" diff, but nevertheless, it needs to be mounted so that the driveshafts are lever when the car is at the normal ride height, and so that when viewed from above, the driveshafts are at roight angles to the car's centreline.
BTW the 7 or 7.5" diffs are measured (the casting thickness side to side) across the front top mounting hole

John

SG,

What I did, was measure the diameter of the wheel / tyre combo (Sierra Donor, so 23") divided by 2 (was 11 1/5").

So the (centre of) the output holes on diff is 11 1/5" from ground.

Size of differential irrelevant - real concern is, as John says, getting holes in the right place. Above will help set the diff height.

ATB

Simon