I hope this thread is still active and not a dead one from 3 years ago. If it is dead, I apologize.
I have to disagree with the hard shifting of a 915 with a Vellios kit. Granted, I have made several "adjustments" over the years to the rough kit, but my track car shifts just as well as my street car with a nicely built 901/914 trans with a stronger intermediate plate. Both run behind 3.2 engines, but the 901 simply can't handle the torque and sticky tires the track car has due to some rough race tracks. The 901 doesn't like rapid changes in torque (slipping or bouncing rear wheels under power).
The 915 so far, seems to hold up better under those conditions.
The 915, however, is NOT bullet-proof. The first Ring/Pinion went south before I bought the tools to set up pinion depth myself. Since then, it has held up well.
That being said, the Vellios kit needs some tweaking in order to make it function as it should (likely most "kits" would, but I have no experience with others).
An alternative that I saw many years ago was from some folks from the Phoenix area used to have an external shift linkage to shift a 915. I have some pictures of such a car.
Click to view attachmentClick to view attachmentClick to view attachment