The 3G Alternator upgrade is a good choice if you have electric fans, or other high current accessories. The 7" model is a bolt-in, but you may require a dual v-belt to spin it. When I installed my first one, it had a noticeable squeal, but only on the initial start up. It was easy to swap to dual belts for me because my engine already had the dual groove WP pulley and 3 groove balancer pulley. I now have a 160A 3G and a serpentine belt setup on my new motor.
Here is a recent thread with 3G info:
http://forums.stangnet.com/762671-pa-performance-3g-install.html