If you haven't got the spring perches welded to the axle housing yet, I'd leave them unwelded until you have the whole thing together, with wheels on the car. Then set it on the ground, rotate the axle housing to get 2-3 degrees down angle on the pinion with respect to the driveshaft, and tack weld the spring perches in place. Then pull it back apart and finish weld the spring perches.
If the spring perches are already welded on then you will have to use angle shims, like the ones you can get from Summit or Jegs, to change the pinion angle after the car is assembled. In general, though 2-3 degrees down angle is what you want on the pinion.
