That'd be a 21 spline. But I've got duals both 21 spline and 40s and haven't had any trouble. IMO duals is the 2nd best upgrade for a toyota right behind 30 spline longs.
He's talking about the output shaft that the drive flange bolts to, not the input shaft. Input shafts are 21 or 23 spline, output shafts are 27 or 30 spline.
