Re: Smog legal 4BBL for 302
Your choiced in carb size sounds reasonable. Most guys make the mistake of going too big for bragging rights, and end up with flat spots when they accelerate from too much fuel getting dumped. The Mustang post seems to be a good idea.
The shift improvers are used to modify the valve body in your trans. The improvement comes from a more positive shift (read a harder shift). Here again is another fine line. A harder shift is better for the life of your clutches. If you go too aggressive, you run the risk of prematurley wearing your servos due to the teeth getting slammed into each other. If you decide to go with an improver, use the "towing/RV" setting that most have.
Limited slips for the rear are quite common. I'm surprised that you do not have it in your vehicle from the factory.