It seems that the neck pocket is wider at the pickup opening end than it is at the heel. I hope that this is by design.
Since the neck pocket and the first pickup hole are joined together you can insert the neck with the heel in the pickup hole. The neck tapers, so it easily drops into the pocket. Then you have to push it into place against the narrowing of the pocket.
It's still too big, but it's manageable.Trying to go butt-end first it was ~3mm to big! The pocket itself has some problems - there is a big tool channel down one side of it and the block is not straight, but I think that it will come out OK it I am careful.
The real problem now is that one of my frets is substantially kinked in the middle - either from an impact or a kink in the fretwire when it was installed in the neck.
It the kink makes it higher than the rest of the frets, I'll dress it and live with it looking a bit funny.
If it kinks lower I'll pull every fret out of this neck and go fretless. They look easy to take out, and I don't have a fretless yet.