You're going to want to mount the box before installing the pitman arm. Read the FSM. That nut on the bottom wants 130 ft/lbs. Good luck doing that on the bench.
Just as an FYI, Redhead has a good reputation, whereas West Texas Offroad delivered unusable crap to me.
I would never go aftermarket on a steering box, but that's my opinion.
That said, after my debacle with WTO I purchased the 100 Series cross shaft, pitman arm, and hardware from a Canadian seller on this forum. I wound up using my original box and swapped the slightly twisted cross shaft out on the bench. No leaks, no issues after several years. Turns left AND right.