You need to keep the OD of the front and rear tire as close as possible. (more)
Generally speaking, if you go wider in the rear (an avergae stagger is 20-30mm) you would need to drop 5% in the aspect ratio to keep the sidewall height the same. The 5% drop in ratio compensates for the added section width. Therefore, the sidewall height on the 225/40 and 255/35 should be close to identical. Remember, sidewall height is a function of the aspect ratio and the width. A 40 series tire will not have the same height in all widths. It is simply 40% of the width.