which rear to use for autox?

Run the 8 3/4 or the 9". Performance will be the same.

To me it's about the finacials and what you exactly have in your stack of rearends. I mean if you have to shorten one and get new axles vs. another one that doesn't need that, that a big $$$ difference. And factor in what rear rims backspace you have on your current rims. Or if you are going to other rims.

I think it's cheaper to find rear disks that bolt to a 9" ford rear. If you are going to buy a brand new limited slip unit (~$500) look into the Tru Track/Quaif/Torsen all geared limited slip. Those are the best for autox and street and road race. And those might be cheaper in 9" form.

Also stick in some axle baffles to keep rear end fluid from coming out the axle seal during high G turns. Those will fit either 8 3/4 or 9" tubes. That mostly would happen on a road course. Autox doesn't have the long sustained high G corners. http://www.summitracing.com/search/brand/Seals-it/Product-Line/Seals-it-Axle-Seals/?autoview=SKU Measure you axle tube I.D. to check fitment.

I wish I had some of those baffles. It will leak out the vent tube when I'm at a track day and I seem to be leaking out the axle ends more frequently. I haven't had my center section out in 13 years to put some baffles in.