Most oldschools and heavy American cars rattle relatively less than imports like Hondas with thin sheetmetal trunks. Chris, a shop in Emeryville CA showed me what they do to decrease the rattling. They do a custom trunk kit with carpet and fiberboard. They clean off the underside of the decklid with a solvent and then lay down Dynamat. Then they lay a foam board over the Dynamat, then a thin particle board over that, then carpet the whole thing. Combined with the carpeting in the trunk kit, when you close the trunk the whole thing seals up real tight, helping to contain all the bass and reduce the rattling.
Just plain Dynamat won't do anything...it's not magic, after all.
The expanding foam that Woodie referred to can also give great results. I have a homeboy who put two 15" subs in a freeair enclosure in the trunk of a Regal, mounted to a custom board and sealed the board edges with the expanding foam. It sounded tight and there were no rattles inside the passenger area, just clean strong bass. You have to be careful using it though. Because it expands, if you use an excessive amount you can bubble the paint. So use caution and do it properly.
Woodie do you have any recommendations for paint shops anywhere in the Bay (or anywhere in Cali). I'm looking to get an '87 Cutlass done. I'd like to get a nice two-stage paint job done. I would like to get nice results but I can't afford no showcar paint...this is my daily cruiser, so I'm looking for something appropriate. Please let me know your thoughts or advice.