Finding a good tuner is very important. You want someone familiar with tuning these engines as they will probably already have stored tunes to use as a base for your tune, and understand what works and what doesn't. They will also be familiar with what the engine can take without damage down the road. An experienced mustang tuner will not only be able to get you the most safe power, but can do it faster which will save you $. Dyno tuners charge by the hour so the faster they can get the tune done the less it costs you.
I was lucky to have a great tuner about an hour away from me who has tuned countless mustangs. He came recommended by several people I talked to in my area. He was even able to send me a temp tune so that I could drive the car to him and didn't have to pay for a tow.(I had also changed the MAF so the car wouldn't run without a new tune)
I'm not sure where you live but good places to get info on tuners would be, the local racetrack(both the track workers and the racers), local mustang groups/clubs, local performance shops etc. The people most involved with mustang performance are usually the ones who know where the best tuners are. Also be aware that the tuners in some areas are being harassed by the .gov because of emissions regulations. Some will only tune a street car if you are referred by their customers. It would be a good idea to get a few people's names and vehicles who refer you in case they have that requirement.