My buddy has the SCT on his 6.0 which is the best, however for the 6.4 the best out there is Spartan tuning.

 

Buddy of mine has a 2008 crew cab on 35's, and he runs a Spartan Tune. That's by FAR the best route to take. He's got a 310 tune, turbo-back exhaust, and Banks ram-air, and his truck will damn near do a flat 8 in the 1/8th. If only a full-size gas crew cab could run that with $1700.

 

while the 310 and 350 tunes are awesome tunes, I have saw cases where the astock tranny lasts less than 24hrs at these power levels. If you want the truck to last trannt wiase I wouldnt advise runnin anything over the 250 on the stock tranny. Call spartan they'll verify this, the engines can handle the extra power but the transmission won't last long when you go over the 250hp level.