The cylinders remain up in the rear deck at the same level whether the car is up or down in the back. With a convertible you really can't have as long of cylinder in the back and have the car buried down. You will probably be looking at a 10-12" cylinder with coil over in the rear.
Instead of the cylinder sitting on top of the coil.....you have a reversed cup that is drilled out and mounts around the cylinder. Then the coil sits on this and is actually around the cylinder. It can drop your cylinders down 4-6 inches with a normal low stack of coils.
yes, most definitely. And you will want the hole bigger then normal because the cylinder will now have up and down movement within the frame. You don't want a squeak. We actually but a 3 1/2 tube in the frame to hold the coil and cylinder centered. Its is also a good idea to have a plate around the tube so that the coil does not eat through the factory spring cradle.