Heres a little trick for you to save on a lot of memory.

Before you make you blendshapes, cut off your head. Then combine the body and the head back again and merge the vertex's. Important - When combining the body and the head, make sure you select the head first.

What this does is re-order the vertex's so that the head uses all the first vertex's and the body uses the higher numbered vertex's.

Now you can make your blendshapes with just the head. When you make the blendshape node turn off Check Topology so that maya will not check the number of vertex's and because you re-ordered all the vertex numbers so the head contains the first few, all will work fine.

Richard