My undercar exhaust car is a 427 so not much will apply, but the frame rails are 4 inches deep. On my car, the way the headers and the collector pipe were fabricated I ended up with about 1-1/4" clearance to the floor with a 3" deep muffler. That put the bottom of the muffler about 1/4" to 1/2" below the frame. As you can see in this photo, the diameter or the pipe you use and the out rigger brackets limit how high the mufflers can be sucked up. I probably would not want the muffler any closer to the floor.
Most mufflers are 4" diameter so the average muffler would be about 1/2" lower than mine and they are pretty visible from the side. If round glass pack type mufflers are used they would be a little less visible under the rocker.