TB spacer = good paper wieght. THey are trash, worthless hunks of aluminum that do nothing other than cause fittment issues with intake systems. Its all marketing, nothing more. I have one on my blown 5.4, its a waste of space.
Intake. The best way to go about this is to remember, intakes are all about marketing as well. They call them cold air intakes, but realistically you're taking the most efficiant cold air you can get (stock intake) removing it and sucking in hot engine air with an open element air filter. You're best bet would be to do this:
Go to a junkyard, get a Mustang 5.0 MAF (mass air flow meter) It will bolt straight to your airbox. Put a drop in K&N (or K&N style) filter in the stock box. Get some mandrel bent 3 inch aluminum pipe and a few couplings and make your own intake piping system.
The Bigger throttle body isn't going to do anything either. Unless you are going to seriously modify your engine, the throttle body is really acting as a restriction, not an opening. THink about it, the butterfly stays closed until you hit the gas, then it opens as you accelerate. Putting a larger unit on there isn't going to net you anymore airflow than you had before. If you really want a bigger TB, take yours off (or better yet get one from the junkyard where you get the MAF off the stang) and use a dremel tool to port it out. It will accomplish the same thing, but not cost you $200.
The exhaust is the best bet you have here. You can add all the components in the world, but a restrictive exhaust will negate all your gains. Depending on emissions, you can hollow your converters and add a flowmaster to achieve excellent flow characteristics. You can just weld in a new muffler in place of the stock one, or get a catback if you'd like but there is no real benefit to the catback, the muffler does all the work.
In the end, you're probably not going to notice any seat of the pants performance gains, and after an exhaust you'll loose gas mileage for a while cause you'll be hammering the piss out of it. But once you get over that, you may see a 1-2 MPG increase. There are a few other potentional freebe mods, but more experience is required for them.
