If you do get the expanding foam, be sure to get stuff that does not have any nutritional value to it. You laugh, but think about it...it will be lots of food for bugs. Before you inject foam, spray undercoating and let it cure. This way, any moisture that the foam absorbs will not rust out body panels from the inside out.
I know there is some special foam that is used to fill in voids in exterior parts, like subframes, which cuts down on harmonics and adds a little bit of strength. I read about it some time ago in Sport Compact Car, though the name of the product eludes me.
Welcome, downwardly mobile yuppies, to the dawn of the Age of Nefarious.
1993 SSEi - FrankenJinx - dead and gone.
1994 SSE (136K strong) - 180* t-stat,HD alternator, most unequipped SSE on the club, XM radio, Kenwood CD/MP3.
2006 Chevy Malibu Maxx LTZ - Custom 2.25" manifold-back exhaust w/FM-80. Probably the only Maxx with this system.