IRQs aren't det for specific drives, but are set for drive controllers, so don't worry about this. Go with CALV's advice.
Try your suggested config. You don't need speed on the 6GB drive anymore, and assuming you don't get burn probs, it's a good config.
<b><font color=blue>~ What do you mean "It isn't working!"...Now where's my sonic screwdriver? ~ </font color=blue></b>