I don't think I had to remove the steering box - if you get the clearance you need you should be able to fish the old bushing out. The fenders are attached to the body and the radiator core support. The mount you're trying to replace is close to the interface between the fender and the body. You'll need to loosen the radiator core supports so that you don't bind the fender when you lift the body. Use 2x10s between the floor jack and the body.
ETA: You should definitely disconnect the steering column from the gear.