damn did you drill the neck holes with a drill bit too small and then driving in the screws caused the neck heel to split ?
Think its a 3.5mm drill I use.
I'd drill a 3.5mm hole for the neck screws and put as much thin superglue down the crack as you can, clamp the sides of the neck, then try and screw the neck screws in. I'd even get a screwdriver to pry apart the crack a little so the superglue seeps in as much as possible.
I did a similar thing to a flying V neck I split the neck heel and superglue saved it !
Be careful to wipe off any excess glue from the crack as you don't want the neck heel to stick to the neck pocket !
Might be better to only screw in the 2 screws on the opposite side of the crack to hold the neck in the pocket while it dries, and clamp across the heel to hold it all together