[Christ]

If you're that serious about losing weight in the car, just about everything can go, honestly. Check out your local craigslist and get some cheap "racing" seats... they're pretty comfy, and half the weight of the stock seats.

The manual transmission is simple, since it's RWD. Any transmission will work, and since you don't need to use a clutch pedal or clutch, the only modification you'll need to make is the transmission mount/crossmember and the end of the driveshaft. (Tranny-side yoke)

If you really wanted to, you could actually put a T56 6 Speed in there, and it would work fine.

If not, any Mustang tranny from the same time period will work, and should match up to the mount location on the crossmember.