Rear seat delete will save you about 40 pounds. Lighter driveshaft, maximum motorsports rear lower control arms, aluminum radiator, sound deadening under the carpet. Foam between the plastic panels and body in the rear less than a pound each. Changing to racing seats should save you 20 pounds or so.
Okay I left out stuff.. I have racing seats, ordered the seat delete, working on the a/c delete now, deleted EGR, no cats, no spare and jack, lighter wheels, aftermarket radio(lighter than the stock beast)..
I will get a tubular k this winter and moving the battery to the rear.
lol on a serious note you could replace the glass with a lighter plastic piece... I priced it out on my Camaro to be around $1500 which is more then the mustang should be and it was a couple hundred pounds...