Well,

I could be wrong, but 6" seems like it is a bit too much to alter a kilt. You could certainly move the buckles/straps, but I would think that much movement would tend to make the kilt look odd. The apron would wrap around the waist almost to the rear.

You might be better served to sell the larger kilts here or on eBay and buying appropriately sized ones. And BTW, I do happen to be a perfect 40!