This day, I have squashed one side of the offending exhaust header.
I had hoped to do the modification in situ but limited access/space made this impossible - pipe removed from aircraft.
I used a piece of tight fitting water pipe, on the side that I did not want to impact/change and Vice Grips to deliver the necessary pressure to the precise point for denting (I did not use the socket). The Vice Grips were repeatedly adjusted & squeezed until the desired dent achieved.
A quick engine ground run up to oil 50C and normal shut down resulted in no marks on the witness tab - Great!. Will do a proper test fly, in about a week.
