Having built race cars for a while I can tell you the Ford 9 " offers more strength than the 8 3/4 , which is virtually identical in size only because there is 5-6 teeth in contact at all times in the 9 " , it also offer the interchangability of the 8 3/4 . The big downside is the parasitic drag of trying to rotate the gears , the old saying is if you see one guy casually pushing a car at the track it had a Dana , if you see 5 guys pushing hard even downhill it has a 9" in it , the 9 " eats up a lot of power . I have always stayed away from the 9 "
Having raced the 8 3/4 it is marginal at best , one the first day I raced my duster & twisted both axles , luckily they didn't outright fail & I made it home but I had about a 20% twist so how far did they twist to stay at 20% ? . I also distroyed the gear set inside the sure grip 4 times , they take the pounding for a while but will fail .
To me it just makes sense to do the Dana as you will spend close to if not as much to build the weaker 8 3/4 unless you need the gear interchangability
