exactly HOW fast do you want to go. You can get V8 performance out of the V6, but it costs approximately as much as an engine swap to a 4.6L and it also depends on how much $$$ you can get the swap parts for. A popular setup from what i've heard is dual exhaust (many companies make dual exhaust conversions for the V6 and it's supposed to add like 15 horsepower), I've also heard of Pulleys (5hp?), and a supercharger will get you up to where the V8's are, but the car will weight less and as a result handle better with better suspension.
Here's an interesting site:
http://www.kennybrown.com/38specialintro.html
I think a few companies make superchargers for the 3.8L.
You can also search the web for V6 mustang enthusiasts, i've actually browsed a few and found there's a lot of support for modding the 3.8L.
Although you may get ALOT of power out of a V6, a V6 will never sound like a V8 so you'll always have the V6 sound.
It all depends on how much you have to spend and what you really want out of your car.
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'84 Mustang 5.0 LX
My car