Congratulations on becoming a Mustang owner!
Assuming you want to keep the AOD transmission, I would suggest an immediate ring and pinion swap to a 3.73 gear ratio. That will net you a very noticable increase in acceleration without messing with the engine or affecting emissions. Costs will vary but the gears are under $200. and installation should cost about the same or more, depending on your installer and the labor rate in your area.
If the gear change doesn't do it for you there are also ways to 'beef up' the automatic transmission and make it shift harder and faster for increased acceleration. That can be accomplished relatively cheaply, too, depending on the labor costs. However, be aware that the Ford AOD transmission in your Mustang is NOT 'heavy duty' and
will break under abuse, which is part of the drawback of an automatic transmission in a Mustang 5.0 and why many owners either go to a racing auto transmission or just give up and convert their 'Stangs to a manual trans. Either way, it gets expensive to replace any automatic transmission so I simply offer the warning that your AOD is vulnerable and should not be abused. That doesn't mean you can't get on it - it's not made of china - but I would avoid power braking, holding it in gear to the redline on a regular basis and manually 'power shifting', all of which will shorten the transmission's life. I would advise you to change the transmission fluid immediately and go to a synthetic trans fluid. I would also consider adding a transmission cooler, which can really extend transmission life.
Hey...good luck with your 'new' Mustang!
