How do I weld a part to the players neck?

So I've been working on a titan from the popular anime Attack on Titan and I cannot seem to figure out how to weld the nape hitbox to where the neck of the titan would be. The titans have random sizes so welding parts onto the neck is difficult.. Help would be greatly appreciated.

Here is the code I have so far which does not work.

 local weld = Instance.new("WeldConstraint") 
weld.Part0 = Titan.Nape 
weld.Part1 = Titan.Head 
Titan.Nape.Position = Titan.Head.Position + Vector3.new(0, Titan.Head.Size.Y/2 + Titan.Nape.Size.Y - Titan.Nape.Size.Y/2, 0) 
Titan.Nape.Position = Titan.Nape.Position + Vector3.new(Titan.UpperTorso.Size.X/2 + Titan.Nape.Size.X - Titan.Nape.Size.X/2, 0, 0) 
weld.Parent = Titan.Nape

Basically I need it to weld where the neck of the titan would be.