It depend on what they are hanging the quilts with. Since you have no way of knowing if 3 1/2 is big enough, I would add a strip of fabric to your sleeve. It won't look as nice as a solid sleeve but if the sleeve is already attached, it would be easier to add a strip to it then take it off and re-do it. I would cut down one side of the stitched sleeve and add a 2 inch strip to it. I would then French seam the other edge of the long strip to the other side of the sleeve.