Help Wanted: Altering a Backless Gown to Suit My Style

I am thrilled to be able to tell you I am going to a wedding soon. I have been waiting for a wedding to come my way for years so you can image how very excited I am! I love weddings... I think. I am not sure actually, as the only one I have ever been to was when I was 7. 

But this one ... this one has to be awesome. My cousin, Scott, and his very longterm girlfriend, Josie are such cool & beautiful people I cannot imagine any thing of theirs not living up to their standards of awesome. Plus, both the ceremony and reception are to be held in late 1930s theater! What could be a better venue to re-introduce me into the magic of matrimony (or at least the party bits of it!)? The theater is gorgeous, from what I've seen of it, and I finally get to get dressed up in period-eque attire and get to fit in with the atmosphere! I am sosososooooooo excited. 

I have a huge problem though... I have no authentic 30s or 40s fancy dress. And I'm far too broke to buy any of the ones I like. 

My (hugely conflicting) solution: Altering a 1980s silk hand-me-down gown to look more similar to the style I like. This could be easy if this dress didn't have an open back. I'm not talking open down to the waist, but open all the way to right above my bum. It is distressing. I am trying to figure if I can craftily add some fabric to the lower bit without it looking bad. I have to go through all of my potential alterations for this dress mentally since the dress is with my mom. It is a problem. 

the back, from what I can recall, is very much like this one. Only not as nice looking on me....

The neck on my dress is high and moderately wide. I think I would like it to be more like this though:

the colors are similar to this dress, as well.

Do you think making the front like this (rather than the 3/4 length sleeves and wide-ish neckline it currently has) would go okay with such an open back?

Now, here are some open back designs I could be very content with if I had a choice (which

A Bit About Weddings

I love weddings. Or the idea of them at least. I actually have only been to one wedding in my life. I was 7 so I sort of hated it. Anyway, I now appreciate them because they are one of the few times large groups of people get together and have to dress nicely, and that makes me really happy. Also, there are some really pretty weddings so I am really excited to go to any weddings in the future (my cousin just got engaged so I will going to at least one soon).
Anywho, I was snooping about in my Grandma Ana's stuff and found her wedding album so I took some pictures of the photos. She was married in '52 I think... and must have gotten knocked up pretty quickly since my mom had two older brothers, and she was born in '55.
... anyway...