Yes, those can be quite a problem to get out. What typically makes them seize is corrosion, gunk, and other crap that's in the threads. Applying heat can work wonders as it softens those materials and can allow it to break free. The trick is getting enough heat but not getting too much. A propane torch would probably not get it hot enough, but acetylene could potentially destroy the head (if you're not careful). That being said, I've always used acetylene for things like this with good results. The plug is steel and the head is aluminum, so you'll want to focus the energy on the steel plug. As it heats, the aluminum will also expand a bit more than the steel, so this can also help loosen the bond. If you've already stripped the Allen, then twisting it hard enough might also be an issue. If that's the case, I'd probably weld a bolt to it with a wire feed welder. Sometimes just welding a bolt to it will heat it enough, so have a wrench or socket handy and go after it as soon as the weld sets (while it's still smoking hot).

If you don't have the equipment and/or the confidence, then it might make more sense to take it to a professional welder and pay him to remove (would certainly be cheaper than accidentally burning a hole where you don't want). If that's a little out of reach, an alternative could be just using that front port on the filler neck. Even if yours has a sensor there already, it's a worthless sensor and it's better to just ground out the wire going to it anyhow.........then you can remove it and use that port. I've re-purposed that port on all of my vans. Tim

PS: There's also the overdrive temperature sensor and that thing is worthless too. If your van is a stick, then there'd be a plug in that port as well (same potential problems as with removing the other plug).