Used LT4's are very, very hard to come by. The were only offered as an option in '96 Vettes, and came standard in the '96 Grand Sport Vette's, which are now collector cars. If you have an LT1 now, you would better off getting LT4 heads, intake manifold, and get the LT4 HOT cam kit. Bolt it all on and your good to go.
