The springs do not sit level in the car. If you measure from the ground up to the front eye of the spring, then measure from the ground up to the rear eye of the spring you will find the rear sits about 7 inches higher.
The correct way to set the pinion angle is with the rear end in the car. Set jack stands under the rear springs so they are holding all the weight of the car,set your new perches on the springs then set your axle housing on the perches. using an angle finder measure the angle of the output of the transmission then set the pinion angle a couple of degees down from that. Center the housing from side to side and tack the new perches in place, then remove the rear end and finish welding out in the open.
I hope this helps, Mike.