Look at the stickers on the radiator cover, it should say 3.8 or 3.9 on it. Underdrive pulleys give 7rwhp, shorties don't give too much, true dual exhaust can range depending on mufflers and cats, but about the same as pulleys probably, intake gives little and a simple K&N drop in usually yields same results, ported heads and intake manifolds with a cam is the biggest rwhp increase, but costs the most and hardest to install. 4.10 gears and a T-lok is the biggest seat of the pants increase, but doesn't truely change the output of the engine, just uses mechanical multiplication to increase accelleration at loss of top speed, plus they can cost a good deal too. A SCT or Diablo handhald tuner is ~$400 and can give a good increase if you want 93 octane, if not they have 87 tunes that still give more power. Throttle body, TB spacer, larger MAF sensor are pointless because the stock ones already flow better than the heads and intake manifolds can support.
www.vmptuning.com
www.rpm-mustangs.com
www.moranav6racing.com
www.supersixmotorsports.com
^^ all good V6 aftermarket places, the last two are more towards lots of power and power does cost.