when you install a 3g alt you need to up grade the power wire........the stock one sucks as it is......let alone putting in a higher amp alt.
this article pretty much explains it all and theres a butt load of posts in here about it, i should know i just made one for when i did mine.....
check out this link
http://www.mustangcentral.net/tech/alternator.html
you can buy an already made power wire from pa performance for about 50 bucks, then the alternator itself, and thats really all you need. the only other thing is having to slightly clearance the alt bracket but....all thats explained in the above link....
it's about 200 bucks for a new alt from auto zone, they were cool and gave me 40 for my old 2 g core. you can buy the alternators else where but i prefered a normal auto parts one so if it went bad i could easily exchange it and not have to send it any where for repair....
ofcourse thats buying stuff new. if you can find a used 3g and or make your own power wire you can do it for cheaper.....