Fog Runner

Ok, I have looked through a ton of threads and have not been able to find the answer I am looking for. I am just looking to level my truck out to get the nose off the ground so I can stop scraping the air dam. I have also called CST, some installation shop that CST recommended I call, and Performance lifts and non had an answer either. I am looking for the minimal backspacing that can be run with the CST 3.5" spindle lift. I know they say 5" but I have seen post on here of people running H2 wheels which is close to 5.5" So why do they say 5" is this to make sure that the tire will also clear. I currently have a set of ATX crawl in an 18x8.5 with a +15 offset, which is about 5.4" backspacing. I was wondering if this wheel would fit beings the H2's fit. How much room is there between the H2 wheel and the spindle? I would be running a 275/70r18 so the tire is narrow, so I shouldn't need the extra space for the tire to clear. Any help with this one would be greatly appreciated. Also if anyone has a picture of the clearance of a H2 wheel with the CST spindles that would be great to see! This is on a 2006 2500 QC Cummins.