I'm pretty sure you're going to be at 35 inches Center to Center, that's assuming that your new front spring mounts extend an inch from the frame rail on each side.
He said the outside of the spring box is flush with the frame.
Regardless of what everyone says it should be, don't jump the shark. Wait until you measure it on YOUR car for real before you start cutting and welding anything. I learned along time ago, every car IS different. Measure for yourself. TWICE. Also measure from your frame to the quarter panel lip. Measure for square also. You don't know who welded those boxes in. Take your time, make sure it is right. You have the chance to center your rear diff in there now, take that opportunity.