The only thing about underdrive pullies that would be negative is the fact that they slow down your accessories to the point that sometimes they cause charging or overheating problems. This will not be a problem if you upgrade to the 3G alternator, and a 3 core radiator and high flow water pump. Even with the underdrive pullies my car charges better than it ever has, and stays cooler as well. And they do add a very noticable difference in power.
Another thing I did to my car that I am happy with is we rebuilt the T-Loc when we did my gears. We also installed an F-150 retainer spring in the T-Loc which is a little beefier than the stock spring so I have no problem getting posi now. Very recommended modification.
And listen to Rick. Anyone who can get a stock Mustang into the 12's is a GOD!
------------------
Gunning for 12's!
Buy your parts here
ICQ# 42269241