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I have welded pins and pulled dents out of headers as well as putting a giant ball bearing on a rod and pounding out dents that way. But….that dent is in the perfect spot to try and use the frozen water method. I have done this when it was -10° outside and it can actually work too well. Pour as much water in the tube that you can before it is impossible to contain it, then lay the pipe in a freezer or freezer chest big enough to contain it. The water will freeze and run out of room to expand as the rest of the water in the pipe solidifies in place. So it will force out the dent in as it is the path of least resistance and since the dent is right at the bend it is in the ideal spot for this method to work.
It will take at least 24 hours for it to work, but you MUST check on it regularly as it can push out the dent and then continue expanding and split the pipe open. I had that happen on a really really rusted header that turned out to be incredibly thin.
You might have to do so final shaping with a hammer/file to make it blend perfectly once it pops out. But it’s totally free, so it’s worth a try.  

wrote:

I have welded pins and pulled dents out of headers as well as putting a giant ball bearing on a rod and pounding out dents that way. But….that dent is in the perfect spot to try and use the frozen water method.

I took advantage of the recent cold weather (6* yesterday morning) and gave it a try. Worked a treat!. There is a small crack that can be welded up but the dent is mostly gone. 
Thanks Daftrusty.
 

I’m glad that worked for you!
That reminds me….I have a Bassani header that has a bunch of dents in the down pipes. I need to get it out and fill it with water so I can freeze the dents out.