I have looked through all my (factory) literature and cannot see any roof aperture measurements. Just out of nosiness, you are fitting a "standard sun roof"----does this mean that you are fitting an "after-market" sun-roof and not the normal 500 roof? Having driven thousands of miles in a 500, in all weathers, I can assure you that the 500 'soft' roof is remarkably water-tight (much more so than the Midget I used to own!). All the after-market 500 as far as I am aware, fit pretty well. When you come to fit a standard 500 roof, the 2 outer rivets on the rear cross-frame are best held down with small bolts going into (sealed) riv-nuts rather than pop-rivets--makes for much easier removal in the future should you have to take the roof off.