Did you ever ask ORD about their reasoning for the shackle flip? I've got a 68 K10 I'm planning on lifting 4-5" as well and have been wondering about that. I'm guessing the springs ride a little nicer with the shackle flip, but that's a lot more work vs. just bolting in new springs. I'm tempted to try the shackle flip and leave the original springs in the back vs buying both.
I'm installing 4.10 gears in mine right now, planning on 35's as well. That's probably a little low for someone who is planning on a lot of 70+ mph hwy driving. 3.73 or 3.92 would be better for an automatic that's going to spend time cruising on the interstate.
http://www.grimmjeeper.com/gears.html is a neat calculator that will let you compare different tranny/xfer case/gear ratios etc, at different rpm's and speeds. It knows what all the common gear ratios are.