the truth about pullies please?

What year is your stang? Since it sounds like your car is stock, you will definitely benefit from underdrive pullies. They help the motor run through the rpms faster for sure. The main drawback is that they turn the alternator & water pump a little slower, so if you are doing a lot of driving in traffic or even idleing at a stop light your headlights & dash lights may dim some (if you're driving at night) and your amps will drop down some. And if it's a really hot day and you are stuck in bumper to bumper traffic there is a chance of overheating due to the fact that the water pump is turning slower than stock at an idle. In my opinion you'll be fine though. It's worth the small risk of driveability problems...& definitely worth the $$!