I strongly suggest the 6.3 drivetrain. It is much quicker than ANY 6.9, period. One minor detail. Either fabricate or use a parts car 6.3 tranny tunnel. Thats a key error that some have made. It's an absolute. 6.3 parts cars are dirt cheap. It's mechanical fuel injection system when rebuilt is bullet proof if you simply change your oil every 2000 to 3000 miles and or every year. Don't believe me, believe the track slips, they do not lie. You would need to replace the 6.9's rear diff, 3 speed tranny, and completely alter it's motor to simply attempt to catch a 6.3. Then the question is still undecided on that computer fuel injection system. I have given up hope with 6.9s and am a total fan of the 6.3.