The fact that it gets so close suggests that it's not fouling on the splines or on the edge of the spigot bush. Is there any debris or grease at the bottom of the spigot bush hole perhaps? A problem I had once with a different setup was that the gearbox input shaft was bottoming in the hole in the flywheel. You could check this with a bit of engineers blue (or paint if no blue available). In our case we just ground a bit off the shaft and the thing was fine until a few years later when an MOT failure due to rust put an end to it.