Place some 1mm washers between the shock tower and mount to help reduce the rear shock tower breakage - this give the tower more room to flex on an impact rather than pushing on the motor plate and snapping...
You can space out the upper shock mounting point by the same to retain the same shock inclination.
As for gear diff, I went back to the ball after a poor start to the season - it was great indoors and when the grip is high and consistent, but I found the ball better on the outdoor track I raced on.
2k-3k for outside use if you wanted a start point...with 3k giving a more "locked in" feeling.
HTH
