Hey Airmale,
My advice -
RAISE the truck 6". 
Just kidding of course. I had a similar pinion angle concern when I I did a suspension swap to raise my rearend. It turned out that the lift made the angle almost disappear. U-joint, hardly has to work at all now.
As for your question, sorry, I have no input, but there are tons of slammed trucks on this site, and I'm sure you'll get plenty of "useful" information.
