1973 Plymouth Duster repair and rebuild

I can not. My drive way slopes down hill quite a bit. The rear tires have blocks preventing it from rolling down hill while the front end is on Jack Stands under the K Frame and A pillars. I was trying to be pro active. Figured set the pinion angle using my digital angle finder and set the perches. Then weld. That sounded easy enough. And would minimize the time the rear end is up with less safety issues on the hill. More so, since I am not on flat ground, I dont want the car way up in the air. Up high enough to get the axle in, then get the car back on the ground.
Whether the vehicle is on level ground or not hasn't a thing to do with pinion angle or working u-joint angle. You can flip the car upside down and take the measurements and come out with the right figures. That is, IF the suspension would remain in the static loaded position. Sticking an angle finder on the pinion with the rear end installed in the vehicle is not reading the actual pinion angle as related to the perches. That's the point I'm trying to get through.