The answer is yes they will move back. Several guys have had the cars up with the shocks off and had the shackles lock over center forward of the mount. You have to make an effort lowering the car to make sure the springs don't do this. Situation normal. The front spring arch is what affects spline and yoke relationship. There are several threads on driveshaft length that you can review and get a comfortable number from. If you are running the seal with the dust boot on the tail shaft then you need to be careful about making the shaft too long. What transmission are you running? The 1/2" should be a non issue if your shaft is in spec.
