Here's the deal, there are a lot of people that think it's easy, these are the same people that use the same shims they took out on the new gears. While this works and the car will go down the road, you're more likely to be buying new gears and a professional install just a few months of hard driving later or a few years if you go easy on it; but you'll be in the shop sooner than that to get rid of the whine. The problem is, when it comes to shims that are 1/1000 of an inch, there might be some adjustments that need attending, otherwise they would have shims that are 1/8" thick.
Most people just don't have the tools necessary to properly setup gears. You can rent the tools at most parts stores, but then you run the risk of improperly using them.
Unless you are positive you know what you are doing, or you have plenty of money and time to burn, I don't suggest installing gears or any other parts yourself.
from what I've heard, AFP is the best in the portland metro area when it comes to anything stang.
I get my trouble parts installed at my uncles shop in Albany, and if he didn't just build Factory 5 Shelby Cobra replica's I would recomend him.
Here's the other thing, if you go to a reputable shop, no matter who it is, they will most likely have a warrenty. If you go buy the gears and have a shop install them, they may cover any problems down the road with the installation. But, if your gears throw a tooth or have a hairline fracture from the factory, you're SOL. If you're going to go with a shop, let them get the parts, they will probably include parts they buy in their warrenty.
Good Luck!