You may want to check with Lloyd Elliot at the link below about one of his head and cam performance packages for your LT engine. Additionally, I believe your Impala LT engine will have iron heads, so you may want to upgrade to some of the GM aluminum LT heads.
Anyway, Lloyd is a really great guy and will give you some good advice on performance upgrades for your LT engine.
Warren,
PS: If you need a set of good quality GM aluminum LT heads, I have some.
http://www.elliottsportworks.com/