What size pulley should I use to gain horsepower on an L67 with an intake. I also am going to replace the belt, and plugs and idler pulley is necessary. I was thinking about going with a 3.4. With the other mods is that going to cause a lot of KR??

First get a scanner to make sure all is and stays well while modding. Then open up the intake and exhaust, 1 step colder plugs, 180 t-stat, no smaller then a 3.4 pulley and see where things are at. PCM would be a good mod as well. And go from there.