I was also thinking that the bowser heads look not right... I think I have not seen any Bowser suits that look 100% right.
I loaded up Blender 3D and loaded in a Human Head and one of the newer Bowser models from some Nintendo games and I noticed that Bowsers head is much flatter than the Human head.

basicly you could let some of your head reach into Bowsers red mane to get the eyes a bit closer to the position of the human eyes... but it would still be not perfect.
I think the neckring with the spikes would still not fit....
I am a perfectionist especially if it comes to Bowser :P

Yeah... I figured that the thing that's gonna get in the way is the jaw after looking up references. I just drew this from memory. It'll be a while before I actually get to even buying the stuff to make it, with all the things I want to complete first.