Your best bet it to go after driveline weight. Flywheel, driveshaft, light weight wheels...
If you can do without the back seat, yank it. If you think spending a few hundred bucks to drop 40lbs or so for a pair of front seats, I would say you are crazy... but if you have the $, go for it.
Are you making a track car, or just a lighter DD?
Weight reduction is expensive when done right, but 01blkGT made some good suggestions with the K-member and
suspension components. Kind of kills 2 birds with 1 stone.