I have an 03 GT and I bought the MM subframes also. I posted last month about sumthing like this where the first shop I took it to quoted me $150 to install it, but when I brought it to them they opened it up and said that it was more complex than any other subframes they've dealt with. They said it is a "superior" piece compared to other subframes they've welded in and said that as soon as they saw the brackets that it came with they knew instantly it was going to be more of a complicated job and that "you have to take out the interior."
So I decided to take it to P&J down here in Buffalo where I live because they are a mustang specialty performance shop and they said the same thing.They've had eperience doing this many times so I better get it done right. I didn't care about the cost anymore. It handles so much better now but I also put new springs, shocks, struts, swaybars, and caster camber plates on it.
At least.....it handled well until I got my car broken into 4 days ago. I'll write a post about that later.

 

There is simply no reason to take the carpet out on a regular subframe install. I have the blue oval ones, and your simply not welding on the floorpan... your welding on the tube support that runs underneath the car. Unless you have the entire thing glowing its not going to start the carpet on fire, if you do have it glowing your an idiot anyway

 

Installed mine with carpet in......No probs!