You'll get lower numbers in fifth than sixth. Diesels are load based engines and need the load from the higher gear to make power. I have seen it first hand as much as 50 to 75hp difference from direct to o.d and we actually showed someone on sat.
Don't forget, Indiana Diesel Shootout will air this Thursday @5pm on Fox Sports Net. For those of you on FB, "join" the event and share with your friends! Lets get everyone tuning in!