Well, sounds like someone already did the big brake conversion. See if you can find a part or casting number on the rotor. It could be GM, Ford, or Mopar, although at this point it doesn't much matter other than it would be good to know if you ever had to replace them. Since we're sneaking up on this as you go I'm assuming you've got calipers and mounting brackets, and they fit.(?). Otherwise, you're back to the rest of what was posted.