9 inch has the third pinion bearing behind the gear. That makes it stronger, but that car isn't fast enough to make the 8.75 shake in its boots.

68 twin turbo Dart 8.19 so far
70 Swinger 360
69 Roadrunner 383 4 speed
69 Mod Top Satellite 383 auto
69.5 A12 Super Bee 4 speed
70 T/A Challenger
70 FC7 Duster 340 4 speed
70 FJ5 Superbird
68 Hemi 4 speed Roadrunner
71 Duster W2 340