Does the Buick pitman arm fit onto the Bronco steering box? I mean, are the diamenter, taper and spline count the same?
If so, what you are doing should work okay. Just center the steering gear while on the chassis and then figure out where you need the master spline to be on your pitman arm so it will point straight ahead. If if aligns with a stock master spline, that is great, but if not then use a file to take out the spline on the arm that is in the way, then install the arm. Simple...