yea the impalas have same size tires all the way around. i would switch impala ss and gp gxp personally but thats just me. the shift kits only cost 90 bucks and are easy to put in. its literally unbolt a few bolts drop the tranny pan. then unbolt a couple more bolts...drop the accumulator. then pull it apart put new springs and seals in put it all back together. and no the air box's arenot the same as i had an 04 gp and there is small differences. and personally i prefer spectra filters over k&n just because you dont gotta oil them and their cheaper. there are new headers in progress of being made that will be strictly bolt on for 350 buck then do 3 inch exhaust from there. im slowly but surly working on modding mine but its a really slow process