Here's a pic showing my drive line angle with 4" Marlin springs, dual cases, and an hp third. My shackle angle looks about like yours, near vertical. This is on an 85, so it's not an SAS. If you drill your shackles, won't that steepen your caster angle? That might lead you down the road of having to rotate your knuckle balls.
![](http://i625.photobucket.com/albums/tt339/piper1961/Truck%20pics/IMG_0091.jpg)