Not necessarily Ted, just illustrating what the possibilities are by mixing the original ratios together. By putting a manual diff's pinion gear on an automatic diff, Mazda ended up with the GTUs's ratio of 4.3:1. To get 3.727:1, you simple do it the other way (auto pinion in manual diff).
in short:
US-spec Auto diff + US-spec Manual diff = one 4.3:1 GTUs diff and one 3.727 diff (7" ring gear family)
J-spec Auto FC diff + US-spec TII diff = one 4.3:1 Infini IV diff and one 3.727:1 diff (8" ring gear family)
FD Auto diff + FD Manual diff = one 4.3:1 FD diff and one 3.727:1 FD diff (8" ring gear family)
Plus, if Aaron Cake can put down 407hp and 360ft.lbs on a N/A drivetrain with open diff for 2 years straight with no issues, suffice to say that it is hearty enough for a 20B running stock twins at least untill the owner can afford a proper TII drivetrain swap. But with a single turbo conversion, it'd probably be pushing a N/A drivetrain too hard as you suggested.
Now all we need to get are part numbers for the J-spec FC Auto diff's pinion and ring gears. Can someone overseas hook us up?
