Totally agree with Grigg, do not weld them first. Finish up the assembly with the axle floating in the perches and get the weight on the ground in ready to drive position.
Loosen the rear u-bolts and adjust the pinion angle to match the transmission angle. Thats all that matters ! Tack weld in place, remove axle and finish welding.
Stacking shims is risky if the center bolts in the springs use a short head. Then you end up with nothing to locate the axle to the spring perches because the shims are taking up all the room.
Go to Harbor Freight and purchase one of these,
http://www.harborfreight.com/dial-gauge-angle-finder-34214.html Its cheap and easy to use and the only tool you need to set up a driveline.
