I am building a completely new suspension for my car. I am at the point of putting the pitman arm onto my new power steering box. What do I need to do to make sure everything is positioned correctly? Thanks in advance!

Look for a "keyway" on both the pitman arm & steering box shaft. It's not going to be a normal keyway like on a crankshaft, but rather a small spot where there are no splines which surround 99% of the shaft & pitman arm hole. Those two non-splined areas are where they mount together. To ensure you don't mount the pitman arm upside down, make the curve of the arm go down & away from the box. Install a huge lockwasher, nut and drive it home with an impact gun. It's as easy as that!

Make sure your steering box and wheels are centered before you install the pitman arm.