No, JLTs are
not carb legal.
Since they are easy to put on and take off, I'd recommend getting a JLT and keeping the stock. Swap the stock back on when you have to get inspected.
Be sure to visit
http://www.retailmenot.com/ and look for coupon codes for whatever website you decide to buy from. It can be bought from
http://www.americanmuscle.com/ for $216.19 after shipping (free), tax (none for me), and discount (6%, use code 'stangnet').
The gain all depends on your current mods. If you have a plenum and throttle body, you may experience up to 10hp gains. Otherwise, maybe 2hp to 5hp.
Good luck!
Oh yeah, and welcome to Stangnet! :SNSign: