Sounds like a lot of work for a minimal gain. A good running 351 can pull almost anything you can put behind it...the limits of the chassis and suspension arrive before the power ends. Remember the 6.9 is not turbocharged.
I would say if the 351 isn't doing it for you then just search for another truck with a diesel already in it. Or the 460 if you don't care about fuel economy. The right gears make a big difference too.