Vintage dress alteration?

I know vintage clothing lovers hate to alter any vintage clothing, so I'm hoping someone out there can help me. My daughter is getting married in June and would like to wear my mother's 1950's organdy formal gown. The problem is that the bodice is too small. I'm trying to think of a way to get it to fit without actually altering the dress too much. I thought of leaving the back open and creating a bodice of some sort and temporarily attaching the dress to it, but the zipper on the dress would be obvious and is in bad shape. Is this hopeless? My mother has ped away and this would be a beautiful connection if we can figure out what to do!