Aluminum help.

I finally got a nice aluminum radiator and had a mishap already. I broke the overflow nipple off in the radiator neck. The nipple was a very lightweight aluminum. What is the best way to get whats left out of the radiator neck?

A square shank screw extractor is what I would probably try. This type, not the kind with threads. Be gentle so as not to damage the existing threads.
https://c.shld.net/rpx/i/s/i/spin/image/spin_prod_1138488612

Looks like it was screwed in? I'd stuff some rags in to catch the debris and allow a vacuum to get it out. Then try the screw extractor as suggested. If it has sealant on the threads it may not want to come out. In that case, drill the stub out with the tap size drill for that thread and run a tap in. It will push the remnants our or re-tap the hole. Have a vacuum running on inside to suck up the chips as they are created.

My thoughts were to drill it out but im not sure how to keep the drill going perfectly straight so I don't get into the threads.

Looks like the same nipple that came in my Champion rad. I did the same thing as you and broke it off. I think I drilled mine out. I can't exactly remember any more. I remember that it was pipe thread and I just happened to have the correct tap. I chased the threads and all was good. I called up Champion and they were really nice to deal with. They sent me a couple replacement nipples.

How is that filler neck held on? I see a gap at the bottom. Doesn't look welded on.

Im not sure, it feels solid inside but there is indeed a gap on the outside like its separate. Thankfully, i used an ez out and it came out. I did not think it would grab it without just peeling it to shreds because its so soft but it did. The radiator company is sending me another tube.