Interior Modifications

Head and leg room for rear seat passengers is severely limited and though the Mustang is classed as a 4-seater, it really should be considered as a 2+2. The rear seating space is barely adequate for two kids. Since I don't have any small kids and I wanted to remove some excess weight, I decided to remove the rear seats altogether. Who needs them in a sports car anyway?
The rear seat back, cushion, and head rest weigh a total of 43lb. I removed the whole assembly and had a custom rear seat delete kit made. It consists of two pieces of 9mm thick plywood that are hinged together so that the back can be folded down, and these are covered by beige carpet that matches the trim at the bottom of the door panels. The rear seat delete kit weighs only 13lb so I've saved 30lb, and I've also gained a substantial amount of storage space in the rear.

The stock leather front seats weigh a whopping 36lb each (without the seat tracks) and lack the lateral support needed to hold the driver in place during hard cornering. Quite frankly they look more at home in a Ford Crown Victoria than in a muscle car so they had to go.
Good racing seats cost at least $300 each and might not be comfortable on long journeys, so I decided lightweight sports seats from a production car would be most suitable but finding them was easier said than done.
I eventually stumbled upon a pair of seats from a junkyard 2001 Jeep Cherokee. They are the correct height and width, the side bolsters are beefier and provide more lateral support, and the seat bases can easily be modified by welding on two steel plates and drilling holes in them to accomodate the Mustang seat tracks. I also had the seats retrimmed in two-tone black/tan fabric to match the rest of the interior. Fabric seats, unlike leather, don't act like a heat sink, saving your back/arse/legs from sweating in boiling hot weather! These seats (with modifications) also weigh just 24lb each so there was a handy 24lb weight saving over the stock Mustang seats.
Total cost (including welding/drilling plates and retrimming) was just $216 so it was a very worthwhile exercise.

The heavy, airbag-equipped stock steering wheel (9lb with airbag) also looks more at home in a Ford Crown Victoria than in a muscle car so I replaced it with an aftermarket three-spoke sports steering wheel and hub adapter (4lb total) that I found in ebay. The new steering wheel is ~14" in diameter so it's more than 1" smaller than the stock wheel, but it's still big enough to see the whole instrument panel between the spokes and the rim. The 5lb weight saving is also pretty handy.
Unfortunately if you remove the stock steering wheel together with the airbag and clockspring, you'll lose the function of the cruise control, airbag, and horn. I'd already deleted the cruise control module previously and I was removing the airbag anyway so they weren't issues, but not having the horn function would definitely be an issue. Fortunately there's an easy solution. There are two separate cables that go to the clockspring; one has yellow insulation around two wires that deploy the airbag and the other has black insulation around three wires (light blue, brown, and black). The brown wire is a 12v live feed for the horn while the black wire provides the ground, so I simply connected them to a toggle switch and mounted it on the underside of the steering column. Job done and the horn works perfectly.