Everyone has answered the questions.
You will need to remove the twistlock assembly from the rail in order to be able to install the ULE body. The best way is to use a vise and a couple of blocks of wood. The wood will protect the rail from metal to metal damage while you press the assembly out of the rail. Put a block of wood on the top side of the rail on each side of the twist lock assembly with a gap between them to allow the twist lock to move into the gap. If you want, you can also use a block of wood against the twist lock assembly. Place the rail and wood blocks in the jaws of the vise so that the vise is pushing on the twist lock assembly from the bottom of the rail and pushing on the wooden blocks on the top side of the rail. The assembly should press out quite easily. If you don't have a vise, you can use a hammer to knock the assembly out. Use the same configuration with the wooden blocks to protect the metal, only with a hammer you will be hitting downward and the wooden blocks and top of rail will be facing down instead of sideways with the vise.
ULE bodies should fit on the classic rail without any size modification to the body or rail. Sometimes you will find a rail that has a pim hole that is shallower than others, and causes the pim on the bottom of the body to bottom out. You can tell when you put the body on the rail. If it rocks or moves when tightened down, then it probably needs to have a bit shaved off. Most likely it will be perfectly fine as is.
ULE bodies require a detent in the side. They use angel threaded detents. AGD detents work really well. When you install them, make sure they don't thread too far into the body. If you can feel the edge of the metal on the detent inside the body, it is screwing in too far. If this is the case, add an additional oring around the outside of the threads on the detent.
Except for the Automag in front, its usually the man behind the equipment that counts.