I know you don't want aftermarket, but alot of the new hemi guys put shorties on them... a little bit of an improvement as far as throttle response, but not like the power increases from having long tubes. to name a few, JBA, SLP etc. You also might be able to get them to fit w/o modifications. The long tubes would be a pain I'm sure, we put dynatech long tubes on my girlfriend's 5.7 hemi challenger and they worked out nice, but I will say they were a pretty penny