The Diablo will be WAYYY more than you could ever use (plus there have been multiple bad things happen to 03 PCM's from Diablos) on that 2003. Hell, even the 3865 will be capable of more than you can actually do to the truck. Two advantages the SC will have over any Diablo for an 03 is speed limiter and rev limiter lifting. Keep in mind, lifting the rev limiter will only show itself if you manually hold each gear, otherwise in drive it will shift at the normal rpm. This may not be bad thing, as you have an 03 and shouldn't be revving to 6500 rpm alot anyway (valve springs). You will have no trans tuning or a whole lot of TM reduction. With an 03 you can actually disable the TM at the TCM. The SC will be better all around for your 03.