dude, what can't be done with a GM A-Body!! You can put dead bodies in those trunks :01nocomment8so: If you take the factory spare out, you can easily get 4 15"s in there. There will be some (OK ALOT of) trunk rattle, but that's what deadener is for.
BTW... if it doesn't already have one, put on a plastic gas cap, and use some clear silicone to fill the gap between the license plate and the gas filler door hinge, and around the tail light sockets. That will help alot.
I've had knock in my 77 Delta for years, almost perfected keeping it quiet