yeah, IB is pretty far from good for SPL. im fact, a small sealed box would yield better SPL than an IB set up.
if youre after SPL, id suggest a large ported application. since your obviously willing to sacrifice all your trunk space, then this shouldnt be a problem, and since you have an old car with a beast of a trunk, you should be able to fit one in there. try a CRX style box, those are easy to build, and they wang pretty good. you should be able to net 5 cubes or more back there with one of those.