Rusty bolt removal Miata

need some advice on how to remove this rusted out bolt. 
 

This is on the rear seat mount of an NA Miata. The head of the bolt is no longer 14mm as they normally are. A 12mm open ended wrench kinda fits on 2 cleanish sides but the bolt doesn't have enough head for the wrench to grab onto to twist the bolt. 
 

I tried hammering on the remaining piece of seat rail but that metal is pretty weak and just bends or breaks off. 
 

I've applied PB Blaster liberally for the last week. 
 

Any thoughts on how to remove this bolt?

 Thanks 
 

I think my first move would be a vice grip. Second would be to cut a slot in it and try to turn it with a flat screwdriver and/or a flat bit in a ratchet. Put lots of downward pressure on the head of the ratchet and see if you can break it free. Third would be to use a hammer impact driver in the same slot. 

I've used a dremel tool with a cutoff disk to clean up the sides and make it better fit the next sized smaller socket.  Try a few heat and cooling cycles to break the rust.  And if that doesnt work, you've cleaned up some metal that you can now weld something onto.

How I would go about this:

1) Throw the twist brush into an angle grinder and clean that bolt head up.

2) Hammer a 12mm impact socket on it, or closest fractional impact socket that can be hammered on.

3) Stick a 3/8-drive hammer impact driver onto the socket.

4) Hit it with the BFH. 

If it doesn't bust loose in about a dozen whacks chances are it won't. So, I'd take the aforementioned angle grinder and cut it out. 

Get one of these and a hefty impact gun.  

  You may be able to shoot PB Blaster under that flange thing; if that doesn't work scribe 2 or 3 lines between opposite pointy corners, center punch where they intersect, then drill the head off.  If there's some bolt left you could try turning it.  If not, keep drilling and then retap the female thread.  If the other one came out use that for size & pitch reference.

Clean it and weld a bigger nut over it. And I mean pour on the heat.

I'm not sure on Miatas, are these through holes where you can cut this thing loose and drill it out and use a nut and bolt - or do the seat bolts go into a threaded (blind) hole? 

Shadeux said:

Get one of these and a hefty impact gun.  

What is that? Looks like it would have a nice bite

In reply to amg_rx7 (Forum Supporter) :

That's just a good old fashioned bolt extractor: Bolt Extractors from Irwin