SO either flange out the top ring of your neck to get a good no gaps connection...

Okay. This sounds promising.

The cavity in the head was created by selecting four surfaces with a common border and then just deleting them. The cavity has no flange whatsoever. Is it possible that by just adding a flange that extends into the head's space, that there would be a surface area for the neck flange to attach to?

The neck is, basically, one big flange. It does taper off and I might be better off if I were remove as much of the taper as possible.

Does the Boolean command work better to join polygons whose faces are similar and parallel to one another?