Assembly question

Hey guys, was trying to assemble the slipperential last night and had a problem where the pin for the drivecup would not fit once the bearing/shim were in place on one side of the diff (the one that goes through the diff from the slipper side)

looks to me like there's not quite enough clearance. Have i assembled something backwards or is this just some fettling that needs doing ? I've tried the cups from the hyper st and the ones that came in the kit.

I did notice that the instructions say to put the pin through a hole, which is not there in the diff casing ?

Is it simply a matter of sanding the surface down to get enough clearance ?