Here's an RB25DET in a 300zx:
It can be done, they love doing hybrid conversions in Japan. Here's the page with quite a few, including another alternative, the SR20DET in a 300zx.
http://www.meggala.com/hybridscss.htm
For actual conversion and costs, you'll have to contact a place that specializes in that.
Here's a couple of good places to contact.
300zx specialists:
http://www.jimwolftechnology.com/
Engine swap specialists:
http://mckinneymotorsports.com/