Ashley I know for a fact Klean Strip rust converter from Walmart (about 6 bucks) works great. It is a brush on but I recomend it. Make sure you remove as much rust as possible and simply brush it on it kills rust on contact.
Had to do the same thing, I didnt have a key either. I made my pans from an old ford truck bed and a audi station wagon. Welded them in. The trunk panel I will glue because of appearance.
I am building a street/strip 1974 dart swinger. So far I have the wood dash trim, gas tank and straps, and rear seat I will not be using. I am sure I will find other parts I will not need sooner or later, Reply if I can help.