You will def need a tune. I would not recommend driving that car without one. You can get a canned tune, a custom tune, or a dyno tune. The cheapest route for a Mustang with basic mods is probably a canned tune. You can find used tuners for like $150 shipped if you search Ebay and Craigslist. Stick with Diablosport or SCT.
Esmeralda - 03 Stang GT: Sniper tune, PP intake/plenum/75mm TB, CAI, Full O/R Exhaust, UDP's, Cobra tank, dual 255 lph pumps, 4.10's, upgraded trans w Transpak, 2400 stall converter, grille delete...
Nadia - 2010 Camaro LS3 2SS/RS: SLP Blackwing, K&N Air Filter, SLP Under Drive Pulley, VMax TB, OBX 1-7/8 LTs with HF Cats, OBX 3" Cat-Back, Custom Predator Tune!!