I did this in 2 different cars.
First time around I used the ae86 filler neck, but it fitted poorly and stopped you using the rubber seal that goes around the neck and seals to the body. This allowed a fuel smell into the cabin.
Second time around I used an rt142 corona filler neck, shortened about 5 inches as its way long, but it does use the internal(to the neck) breather that the ae86 tank has (ke70 is external breather and the ae86 tank isn't equipped for the hose to reconnect). I went to the exhaust shop and had a 5 degree bend put in it half way up. The advantage of doing it this way is the fuel tank has the same cap and neck diameter as the ke70, so it looks 100 percent stock and you can still use the gasket. I installed the tank then pushed the filler down through the hole and straight into the tank. Fits very snug, works like factory and no fuel smell as you do not have to leave the gasket out. Also, there is no modification of the shell involved, as last time I had to tin snip some metal away around the filler with the ae86 neck.
Hope this helps.