i have a 99 gtp w/ 135000 miles .what mod are best bang for the $ can i put on a 3.5 pulley if i run 93 octane .does it do any good if i dont get more freeflowing exhaust? i did read the post safley modding 3.8 but i also read you can run a 3.5 pulley if you use 93 octane