How to remove Corroded Exhaust olive/donut

It looks like the backbox has never been off my 83 Mondy, the exhaust donuts or olives (whichever you call them) were basically fused to the intermediate or test pipes (presumably in the US you have cats where these short central pipes are), anyway I managed to break most away and chip one completely away with a screwdriver edge and hammer, the pic on the right is the one I managed to get all off of and I'm holding it there for comparison, the left one is so heavily corroded its become part of the original, I broke two drills and wore out two Dremel attachments trying to remove it.

My only option I can really think of is to grind one edge of the 'new' donut down and basically try and marry it up to the bad part, although being a perfectionist (an annoying obsession) I really wanted it all out

there are two small cuts in the left one presumably from a failed previous attempt, but it doesn't leak

Any ideas???
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I would if it hadn't become fused to the flange, I tried drilling it and it was rock hard, one problem I have is theres too much spring in the exhaust, when I hit it most of the blow is lost, the problem I have is if I remove the intemediate pipe and the other end flange is just as bad its behind the wheel where the rear bank comes down and even harder to get to, so if that one is fused I would have an even bigger problem

 

I've already got the new Donuts from Superformance here in the Uk, I did contemplate removing the exhaust but the rear bank is so hard to get to I thought otherwise, maybe I'll see if I can wedge a lump of wood between the pipe and chassis and strap it up somehow, then I can hit it better without movement