worst case is you contaminate the oil, give up and stick a cork in it, run it and it fails. Anytime you open something up, you're taking a chance, so don't worry about it.
I'd weld that bugger. Rig up something to hold a washer, or nut over that hole and weld up the center of the hole. All the way from the bottom to the top of the nut. Ideally you'd step down the amps so you don't melt the whole nut off finishing up, but you can stop welding at that point, let it cool a second and finish.
Heating with a torch will do the same thing. Not a small propane torch, you need to get it hot fast so the metal upsets and shrinks when it cools, this is what you're after with welding, not getting something bigger to grab.
IF you drill it, I wouldn't use an easy out, I'd drill it through, use a hacksaw blade or jigsaw blade and cut through the threads, then collapse the shell with a pick or small chisel. Use a drill just smaller than the threads, and saw about halfway through the threads, you'll see them easy enough as you cut, by hand.