I'd bolt it back up in the original location. The more shackle angle you run the more the spring rate rises as it compresses. Imagine if instead of having a rear shackle, it just had a roller. In that case, there is no force on the spring eye other than up and down so as the spring gets long the only forces in the system are still just the rate of the spring's steel being flexed up. Same as the spring goes into a negative arch, you're still only flexing the spring material. Load and deflection measurements of this system give you the spring's true rate.
With a shackle in the system at a severe angle, when the spring goes flat and tries to get shorter it has to pull the bottom end of the shackle forward. In order to pivot forward the shackle has to become more vertical so it's lifting the truck at the same time as the spring itself is trying to lift up the truck. This causes a skyrocketing spring rate. It's enough that when we've played with proving the concept an extreme angle will make the truck feel like it's bottoming out when the spring goes flat. Same truck with 20 degrees less shackle angle and the ride quality improves. In general a rising spring rate is desirable from a load carrying standpoint but you have to be careful how much it rises. When your shackle angle (measured between spring main eyes and shackle centers when the leaf is flat) goes to 60 degrees your suspension rate can end up 50% over the spring's rate due to the shackle "standing up".
You are set up to go down to around 55 degrees with the flip moved forward. If it really is moving more freely (which does show up on a graph but not in a huge way) at the static ride point you're testing, it may not show up as well in the real world because of the aggressive rate rise as the suspension compresses. You're at a slightly softer point on the curves now but it gets stiff quick. Keep in mind this rate rise isn't only when the leaf goes flat, it ramps up to that point. I understand that factory 3/4T pack will probably just make flat or maybe a bit inverted but the effect starts at ride height.
Part of the wonderful of the tension shackle is that the suspension rate increases dramatically BUT it's a much sharper increase in rate and happens farther into the travel so you get a suspension that protects itself from overcompressing aggressively but lets the axle travel pretty freely (at a rate very close to the springs' actual rate) up to that point. It's almost a bumpstop effect built in which is why the OEMs love them.
You're actually in a good position to test this out if you want to play with it. Drill some extra holes that give it a more extreme angle (around that 1.5" ahead looks good for this purpose) and drive it a while. Then slide it back and see how it feels. Fortunately on a truck it's easy to do since you don't have to drop a tank to get to the inside of the frame. I'd be curious to see how noticeable the change is on that combo of parts. If I had to do it once I'd follow the book.
