Yes there is a difference. What we did with my car was weld/graft in a donor piece from a non A/C car into the firewall. The two mounting studs for the non A/C heater box on the passenger side are a direct bolt in. The mounting stud on the drivers side of the heater box is off about a half inch or so. You will see an indent in the firewall, or at least mine had one. Drill your new hole for the mounting stud there and it will bolt right up. I went this route because I had a working non A/C box, but was missing everything for the A/C set up.
