Kitbashing: seam between neck and torso

[Megakyle]

I've frankensteined a little before, but only by very closely following tutorials. This opening gap appears during animations, but isn't visible during A or T posing. Maui's great guide seems to imply that the problem here might be weight painting (but both are the same deep blue color) or perhaps the vertices not matching (however they do seem to match since the default poses match perfectly).

I've had very little time with Blender, I hope it's something simple I'm missing. Any advice or things to look into?