So there are a couple of named Fiend leaders that will give you their head after you kill them. What can I do with these?

You turn them in to to Major Dhatri at Camp McCarran.

They are for bounty for the sarge in Camp McCarran which is a huge NCR camp with long concrete wall west from Freeside East Gate. You get paid extra if heads are in good state (so don't crit those guys too hard).

If you've killed cook-cook you should go tell Sergeant Betty too.