If your mustang has a long steering shaft(shaft goes directyl into steering box) this is what you need to do. Remove steering wheel, remove bracket holding steering column underdash, pulll column up thru fire wall.
take off pitman arm, if you have a stick shift remove the clutch equalizer bar assembly from the frame. remove the driver side motor mount jack engine up about 2 inches to clear manifold. unbolt the gear box from frame. Before you start pulling it out put a plumbing pipe foam wrap around the shaft in the car to prevent dash board damage. Start to wiggle the entire shaft out around and thru the power steering lines if you have power steering. Be prepared to do a lot of cursing , the easy part is taking everything apart. Taking it out and putting it back in will drive you crazy, not much room anywhere between the V8 engine and the frame where the box is attached. Put it all back together the way you took it off and your done . Good Luck!!! and get a wheel alignment when you are completed.
Since you have one motor mount out already you may as well replace both with new ones..