LOL! I've raced Pintos since 1975 and haven't seen a 63/4 lsd since about 1980. Put a 8" in and forget it. Woun't slow you down one bit, and their bullet proof. Get one out of a v-8 Mustang 2 and it should have a 3:55 gear in it. Try it/you'l like it.
My 6 3/4 is a "Lincoln Locker" and probably not the best for cornering.
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OK, I'll bite...what is a Lincoln Locker?
My first 6 3/4 was locked (in despiration) with DevCon "iron", packed into the bowl of the carrier. Axles were put in place and the "stuff" was allowed to set. Used it for a driver's school at Summit, and races after it. Never found any iron in the diff oil after that.
Eventually, I found out how to lock it with welded spiders that were placed into the diff during assembly.
Welded diffs in a rear drive car (I can only speak about solid axles) require an adjustment in technique, but very driveable.
I've only seen one LSD, which I sold to a Saab guy that eventually used the internals to fit into his own carrier. I believe the axle spline counts were the same.
I though the 8" only came in the wagons and some of the V6 models. I don't think it is legal in the 2.0 and 2.3 versions. It is also heavier which makes more unsprung weight and more weight overall.
Carl