A-Body Shackle Angle

Hi,
I´m about to take measurements for a new drive shaft. Body is in the rotisseries, and I can´t put any load on rear axle. Will the rear spring eye move 4” to the rear, to put the shackle in correct angle, when car is build and on the ground? Leaf springs are SS 002-3
I don’t mind put ½” spacers with the front hangers, since this will give me an additional ½” for tire clearance. Car is ´68 Barracuda.
What if I build the driveshaft to original spec. which is 44.8” in this application, but decide I need the spacers, and have to pull the drive shaft out ½”, will this affect the strengths of the spline engagement with the slip yoke in transmission?

I have tried to explain this to the best of my ability, and hope for your experienced advice.
Thank You
Stefan
20160625_161838[1]ShackleAngle.jpg