i think the atlas 4 spd is 1'' shorter than the np231 with sye, the klune + D300 (or D20 like Scott's) is about the same length as the 231 if i remember correctly
you can't reuse the stock d/s since you'll need a cv one and you also need splines between the yokes to address the up/down movement of the axle (so that's a whole new d/s like he ^ said).
my d/s came to 21'' long between yokes (or 21.5, can't remember exactly, i had both lengths written down but dunno what it came to, I have 7'' travel splines so not really important), i think the klune is 6 or 7'' long, the atlas 4 spd is probably an extra 4'' compared to the D300 but i think they have the super short output (I have the long one due to the VSS gear) so depending of what you go with you're looking at a d/s between 15 and 18'' long from yoke to yoke.