Believe me, if the man loves you and wants to marry you, I can guarantee he will only notice the glow of you...really, the dress doesn't matter.
I didn't even wear a traditional wedding dress, just a floral dress that I got off the rack, also to save money. My husband married me (just 10 months ago), and not the dress...really he couldn't have cared less what I was wearing, as long as I said "I do!" I focused on my hair, makeup and accessories (made my own crystal necklace and earrings for about $18 dollars) and worried less about the dress.
But, I do understand, you want to be beautiful. for him...If it were me, I would use some lg safety pins and overlap the corset in the back and pin it..have a friend help you. But not too tight or it could come undone....same with the slip. The fact that the dress is a little big, will make it less easy to notice that there might be some gathered areas where you have pinned. Not pinning too tight will also keep from making you look smaller and therefore the dress bigger. For the dress, I wonder if there is a way you could get some wide ribbon (try it at home with like a kitchen towel, folded) and tie it around the waist to make the short torso less noticeable, give you more of a waist to match the dress waist. Adjust where the ribbon sits to make you look your best. These are things I would try..it could help just enough to make you feel better.
Let us know how it goes...stay the "happy girl". Best wishes.