It can be done with the subframe on, it just requires careful manuvering of the rack's body around the brake booster and the brake hydraulic lines.
If I did it again, I'd leave the subframe there and just be patient.
OTOH, if I needed a clutch and a rack, I'd definately do both at the same time as the rack replacement is FAR easier with the subframe dropped.
The procedure is messy and mechanically challenging, but doable by anyone with a decent level of mechanical skill.
Prepare by cleaning the underside of the car. I drove the car up on ramps, sprayed the entire area around the rack with engine cleaner, then power washed it from the inside of the engine bay and from underneath. The more stuff you clean off early, the less that will drop in your face while you're lying on your back.
Drain/expell as much of the fluid as possible. I disconnected one of the hoses from the cooler, drained it, then cranked the engine with the DIS unplugged to get as musch fluid out as possible. The more you get out at first, the less there will be to drip in your face while you're lying on your back.
Stop by the Ford store and pick up a couple of extra Teflon seals. Your replacement rack should come with 2, but get a couple extra. They're about 75 cents each.
To disconnect the fluid lines from the rack body, go in from the top after removing the air inlet duct and anything else in your way. It's much easier that going in from the wheel well.
Make sure you purge all the air from the system after reinstalling and sealing the new rack.
If you don't have the step-by-step instructions in the Helms, better get those, too. They will help keep you on track.
Good luck.
Steve