Minecraft wiki entry for Locations, associated text: In the large fortress which engages into the cliff wall there is a corridor where you can hear the sound of the Nether. In the ceiling should be a lever to open a piston door to expose the Nether portal.
The hill where the large fortress is built is joined to a second hill. Deep under this second hill is a massive stronghold connected to caverns that lead to the large canyon. It is possible to access it from the canyon, but requires quite a bit of work. The stronghold contains at least 3 chests. One in the upper chamber, and two deep below in the library. End Portal Frame at X: -338 Y: 21 Z: 306