1) If you remove the sound deadener from your car, it will be very loud. It also acts as a insulation. Your floor boards will get very hot from your exhaust and motor. Not a good idea. You will save about 30 lbs. Not worth it.
2) The lumbar seats only weigh about 5-10 lbs more then the "normal" Mustang seats. Once again, not worth the weight. Racing seats are comfortable at first, but if you sit in them for 30+ mins, you'll quickly regret you bought them. If this is going to be a daily driver, stick with the stock ones or get another set of stock ones. The stock seats with lumbar weigh about 35-40lbs.
3) You have about 4 inches of room to work behind the little "wall". They do make a gauge pod for there. Check out Autometer.
4) Your on the correct road to a 13 sec car. Weight is a big factor in it, but if you remove 50lbs, you won't see that big of a difference. It's better to spend your time and money one something else. Full exhaust, 3.73 gears, Subframe connectors, Cold Air Induction, Bigger Mass Air Meter, 65mm Throtle Body, if you have an offroad H-pipe you can get rid of your smog pump. You'll save a lot of weigh if you get rid of your A/C. With all that, you should be very close to high 13's. Once you get more money, get different heads, cam, and a set of roller rockers.
Hope all goes well with ya.
