A lower back spacing number pushes the wheels out further. The higher number will have the wheels/tires closer to the frame, causing the interference issues you are worried about.
2010 JKU. 3.5 RK Flex, 37" Nitto Trail Grappler MT, Method 105's. Poly Drag link, Fusion 4x4 Tie Rod, DIY Hydro Assist, Ultimate Dana 44 Front, Rubi 44 Rear. Ten Factory Chromoly Axles, EVO RockStar Skids 5.13's, VKS Shorty V2 with Warn HS9500i, EVO rear D-ring mounts, VKS Prerunner Sliders, Synergy Cage