gas tank filler neck issues

I am working on a 71 demon that came to me in boxes. so far things are coming together, but noticed that my filler neck for the gas tank is different than others. Instead of having a mounting flange with 4 traditional holes in it, it has 3. and they are not screw holes, but more like notches. do I install the neck through the body first then into the tank mounting it from the outside of the trunk, or install it from the inside of the trunk? My 67' dart mounts from the inside and has 4 mounting holes. Any help would be appreciated.