Placing blocks on top of custom blocks w/ custom model

I am creating a Halloween mod and my current struggle is perfecting Frankenstein. To spawn him in as a mini boss you have to place Frankenstein's head (a custom block with 1x1x1 dimensions and custom model) on top of his body (also a custom model, but with bounding boxes set to 1x2x1). When I try to place the head on top of the body it doesn't allow me to. If I enable gravity on the head and let it fall on the body, the head breaks. Could someone point me in the right direction?