I messaged with AMWorks again, and was told the GT12/V600 transaxle is exactly the same one as the V12VS manual (the shifter is unique to the car, obviously). The 'carfection' video on the V600 said that the final drive was shorter, but it might have simply been wrong. Looking at the drawing posted above, it looks like it would be hard to change the final drive without changing the entire shaft because the pinion gear is ground into the shaft, so that would seem to be a big change...