What exactly are you doing that may need a different length of shaft? I swapped my 3 speed manual for a t350 auto in my '63 and all I had to do was find a front yoke that had the fine splines, the u-joint was the same size and the driveshaft was the right length. Earlier model driveshaft (mine) with a later model yoke (fine spline). I found the trannies were both the same length, atleast with-in less than an inch. So for me it was an easy swap.
