Dog/Cat Introductions - How to curb aggression

My partner and I just adopted a 3-year-old Pembroke Welsh Corgi we named Hansi this past Tuesday. He's currently the only dog in a home with four cats. :-P

We have been taking the introduction process as slowly as possible, keeping him out on a leash during lunch and at home during the evenings (he's crated while we're gone) and correcting him with the "Leave it!" command whenever he makes a move toward a cat. Sometimes he'll be fine, but other times he reacts with what appears to be aggression - loud barking, growling, and lunging, ESPECIALLY if a cat approaches him while he's crated. He usually reacts with mild curiosity while he's out with us.

Does anyone have any suggestions on how to continue with the process?