Brother is driver I am not sure exactly what the trap rpm was. It is 4.10 gears 28/9 Hoosier bias ply slicks at 15lbs, 205/70/14 radials in the front. MPH was 113.5. So if I assumed 8% converter slip would be 6k at that speed.
Your math is pretty good, but that bias tire is going to grow a little bit too.
8% is too much though.. if the trap RPM is 6k. Like I said, not a fan of shelf converters...