You have my sympathies Jason. Been there and done that one. Those are real pigs to get at. You might be able to chase it undone with a chisel? Lots of plusgas (not WD40) essential. Oddly, although I have a later watercooled one, I had exactly the same exhaust fitted and after doing battle for an entire day, went to bed and slept on it.
In the morning I decided to ditch the whole silencer and fit a Kawasaki one so I never had to tackle one of those blasted tail pipes again. They really suck!
https://club8090.co.uk/wiki/Exhausts_Custom" onclick="window.open(this.href);return false;
The only other way is to find a way to destroy the nut, but I dunno, by the time you've got a rusty box off and messed with drilling and new gaskets everywhere, you may decide its simpler to do what I did. Runs better on the kwaka one too and £25 to replace each time instead of a £100+. All kinds of funky exhaust cans you can hang on those headers.
You could try supporting engine on a jack and see if removing unbolting engine carrier each side + lowering slightly will give you a smidgen more room? I think thats what I had to do but it was several years ago. I know an angle grinder featured heavily.