Best way to tackle this job?

I understand where Scampman is coming from, and I understand doing things a little at a time so you make sure to finish them. In this case though, I would at least do the headers and steering box at the same time. You will likely have to raise the motor for both jobs, so why not do those two things at the same time and only have to raise the motor once.

I would put the car up on the ramps, separate the pitman arm from the steering box output shaft, unbolt the motor mounts (and probably just loosen the trans mount so you don't tear it). You may have to disconnect the trans linkages so you don't bend anything. Find some way to block it up though so it doesn't slip and fall on you while you are in there working. Then jack up the motor and drop the driver side header out. Swap out the steering box and put the new header in, then change out the passenger side header.

If you can get away with only pulling one motor mount bolt, so it can pivot on the other, do that so the motor is more stable while you are working. Don't know what engine you have, but when we put the fast ratio steering box in my Barracuda (with a 400 in it), we had to undo both motor mounts (and remove the driver side header) to raise the motor enough to get the old steering box out and the new one in.