You're correct 3.31 is standard in 5.0 manuals. 3.55 & 3.73 are optional. In so far as I know if you order 3.73's you also get the torsion diff. Normally 3.31 & 3.55 come with a standard clutch type limited slip. The standard limited slip is probably better for drag racers although really serious drag racers will want a spool. The torsion is preferred by autocrossers and road racers. IMO the 3.55 is the "Goldilocks" gear. The MT82 used in six cylinder cars has shorter gearing than the V8 models hence no need to offer anything shorter than 3.31 in those cars.