6.0's are the easiest as they are considered "hot/tuned" from the factory -- so a few add-on's gets you quite a bit
the 01-03 7.3's have PMR rods in them, not forged like the earlier versions - soo 400 HP is roughly the limit on those engines w/o changing the rods
and for the 7.3's - Wildman's tunes are some of the best! There is also Jody DP and I think Eric @ Innovative that custom tune them
(although I will be purchasing a Wildman chip soon
)
EDIT: in reviewing the posts while I typed the above - I think if you are going to be going for that sort of HP - it doesn't matter which one you go for
