I have only done one of these. It should fit snugly, but if the neck is too wide you don't want to force it.

Sometimes the slots are slightly trapezoidal. If that's the case, instead of trying to slide the neck in, you might be able to put the heel of the neck to where it touches the bridge side of the pickup cavity and drop it in. If that doesn't work, then you'll need to sand the neck with a block...but just enough so that it drops in from the top. The joints are made to be tight, and erring on the side of slightly wide is better than slightly not-big-enough.