If you use O36, you can get an O27 loop inside. You will have to do a little "adjustment" to mate up the switches, but that's something that's been done before. For O36, you'd probably be using Fastrack as I don't know of any tubular track of that size. For the outside loop, by the time the roadbed is considered, you need around 39" in width, and that's with the track right to the edge, so you probably would want a bit more space.
