From the research I've done, it seems the spline count is the same, but the spline angle is different between the 727 and 518, so to do the swap you'll need a 518 slip yoke. I've noticed some aftermarket yokes are advertised to work for 727 and 518, but I'd bet they just machine extra spline clearance to work with either. I'd be uncomfortable using one of those.
I ask if there was a slip yoke spline difference between 727 and the 500/518 ....no replys . I read this a while back. Maybe it was incorrect, not sure. So what have you guys been using (that has not created any problems) for the driveshaft slip yoke ??
When I installed the A-500 in my Duster I used a 727 slip yoke. It is my understanding the A-500 and the A-518 both use that same 727 slip yoke. I have no first hand knowledge of the A-518.