You could switch to a fiberglass subwoofer enclosure (I could build you a tiny lightweight one that would outperform anything you've heard)locate it just behind the seats and move the amp and other gear to under the seats or dash, one doesn't really need (or want) rear speakers in a two seat car, so you could get rid of those an in lieu of them install super lightweight, horn loaded, super efficient speakers in front(I also have a lightweight fantastic design for these).
I keep hearing conflicting reports about LS weight. One source said the block alone was 450lbs(the quoted weight of the L6s) and others claim that's the entire engine. Based on this I've decided on a L28ET build, but if you've actually weighed your car with both configurations that can't be the case. I am seriously reconsidering my options. I've just realized how old this post is, who knows if you even still have the same car.