No worries about the questions - it's rare that I actually have more answers than questions. I recently did a bunch of research about shackles, though, so it's all fresh in my brain.
This page has some great information regarding shackles. My abbreviated version is this: shackles change the geometry of the rear suspension and
tend to flatten an OEM spring pack in a spring-over-axle set up, thanks to leverage. They work best with an after-market spring pack that is slightly longer than the OEM pack. In general, though, an AAL is going to be a better improvement than a longer shackle.
See my attached images.
You can see how my original leaf packs are slightly inverted, or bending downward. This is also
without a load in the rear. Incredible, huh? This is the result of those longer shackles.
I am installing that AAL I mentioned (same one in your link) and will test it in conjunction with the longer shackle, as we've got the Eezi-Awn mounted all the time and a custom bed cap (more weight than the average truck). According to Automotive Customizers, their AAL improves the load capacity, but that was from a person over the phone who had to ask someone else. I think that information is fishy; no mention of how much, but they probably just mean that the spring packs won't compress as much with the same load. We'll see how it works out.
Longer u-bolts, yes. They ought to sell them with the packs, but you should also be able to get them from auto parts stores, a spring shop, off road supply, or perhaps a specialty hardware store. U-bolts aren't that difficult to find. Obviously, they just need to be the same width and then you trim off extra length after the installation is complete.
I suggest, though, getting this stuff through PRG. Just an opinion.
Edit: I cut the tailpipe, too, in order to make space for the longer shackle to swing backwards. I don't know if that was totally necessary.