We've found that the Zone 6" kits won't really clear 37" tires without trimming, and we don't really recommend stacking strut spacers. If you wanted to take this kit higher, you'd be better off grabbing a stock replacement strut or coilover to raise the truck up by a slight margin. As far as your backspace is concerned, Zone's recommendation of 5.50" of backspace is more concerned with knuckle clearance for 12.50" wide tires. If you use a tire that's narrower than 12.50", you won't have an issue, though you could use wheel spacers if you really want to use wider tires.