I just switched from stock tires and rims to 14" ITP C7's with 27" MudLite XTR's (see picture in my sig). I believe the offset is different as my rig definetely has a wider stance now - about 4" in the back. Of course, it may be due to going from 12" to 14".
itp rims have a wider offset, don't have the numbers to prove it but have the stockers and itp and can definatly tell a difference. might want to call itp up and ask them directly, i would be that they would even know ac's offset.
you will be 2 inches wider than stock on the rims and if you change your tires from 10in wide to the 11's or 12's wide that come on 26in tires and bigger you will be at 4 to 6 inches wider than stock so itp rims with 26x11x12 tires will put you at 52in wide compared to 48 stock. The 27x12x12 will put you at 54in wide in the rear.