Shouldn't be hard. You'll need a brake booster, and a few other parts. You'll probably want to change over to disk brakes aswell if you don't already have them. There are guys here that have done this, I haven't so I don't know the specifics. But it can be done.
I would recommend converting to a dual master cylinder at the same time. You should have plenty of room with the 6 cyl. I would look for something off of a 6 cyl Maverick. That should get you pretty close. You may have to change the brake pedal, also.
My wife says if I buy another Mustang, she's gonna divorce me. I'm sure gonna miss her....