[felippe]

Here's some actual links you can click on and take you directly to a few sources:
http://www.3.8mustang.com/wiki/Main_Page
http://rpm-mustangs.com/home.php
http://www.v6mustangstuff.com/

Good luck!

 

[stanwyk55]

americanmuscle.com
lmperformance.com
mustangsunlimited.com
supersixmotorsports.com
chipsv6specialties.com
tmaturbo.com
brothersperformance.com

 

[wetherly]

Clifford where 6=8 ..
http://autos.groups.yahoo.com/group/6cylMustangsonly/

 

[Keme]

Hennessey is very good for muscle, I know they sell kits, im not sure about for 2003, but they might.