I was concerned that with the 6.25" backspace, the inside of my tires may rub the wall of the cockpit unless I use a spacer to push the wheel away from the hub. Am I thinking backwards? There's a good website for helping you figure all this stuff out. The hard part is trying to decide whether to add or subtract the different numbers to see if your tires will be centered in the wheel well.
http://www.rimsntires.com/rt_specs.jsp