How many using an A500 overdrive ?

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.