Shorting an axle and measuring for axles?

Perfect scenario, empty block and trans mounted in the car. Use a laser to centerline the rear yoke through the block-trans. To elaborate? Center the yoke in the tunnel center. With as much movement the rear will make, what's a few degrees off side to side? Drop a plumb bob from the yoke center (knowing the offset of the 9" pinion to center) then plumb bob the frame (to marks on the floor for both frame, pinion and offset of rims-tires). There's your blueprint. Axels can be "close" since spools and axels have plenty of spline for play. Rear attachments are the last thing done IMO. Tack then Weld in a jig to keep housing straight.
Use the wide masking tape on the floor for your marks, car can move but they don't.
Tape is also used to center-square (making a T with the top line 90 degrees to the other) the whole car when ready to setup the car.