| Whip out the intermediate pipe, clamp the nut on the inside (molegrips??) Then drill the top off the nut like you would a pop rivet.
You should then be able to remove heatshield and pull the rest of the nut & bolt assembly out.
The drilling bit's probably going to be easier if you can get access to a bench drill, as everythings clamped in place then. | 
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