I managed to get all but one bolt out without breaking them. How would one remove the stub shown?

also, how would one clean up the threads to prep for new bumper?

 

Douse it with Kroil (preferably for a few days at least) and use a center punch to put a dimple into the middle of the broken bolt. Then carefully drill a hole big enough to get an extractor into it and carefully work at twisting it out. For the others, run a properly sized tap through them.

 

This!
Plus, don't try to rush the process. I would usually say use some heat on it, but your gas tank is right there, and telling someone to do something that might not understand certain things could be dangerous.
That is why battery manufactures have to put a label on the battery that says "CAUTION Don't drink battery fluids"

Also while your at it take a wire brush to that rust, and spray some paint in that area.

OH!
One more thing.
Look on the back side of that. The broken bolt goes all the way through, and I bet you might be able to get a pair of vice grips on there?

 

One more tip on removing that broken bolt...yes, center punch it, try to get it as dead center as possible. I suggest taking an extra few minutes with a dremel and a burr bit to get the working surface smooth, and not jagged, it will be easier to punch in the center. Once punched, use the smallest drill bit possible and drill a pilot hole, be careful and make sure it's as straight as possible. Slowly step up drill sizes until you get to threaded hole. The more material you remove from the broken bolt, the easier it will be to get an extractor to bite, and the heat/vibration from drilling will definitely have helped for the extraction. Chase with a flat bottom tap once it's out.
If something happens like breaking an EZ out, come post again.

 

You can also soak that area with PB Blaster if you can't find Kroil at your local auto parts store.

 

7/8 is pretty good. I say forget about it

 

Thanks everyone so much for the input. Ended up being able to coax it out enough from the rear with these bolt extractor things i had enough to get a bite on the front. Plenty of PB blaster was involved.

Here is what I used. For future generations reading this from Mars. Only took two hours. lol

https://www.lowes.com/pd/CRAFTSMAN-6...Set/1000594521