Dave, cut the exhaust nuts off. If you have never moved them and you do not know if the previous owner ever has, then you could strip your threads on your heads. They could have been fitted for 10s of years and be pretty much 'welded' (through corrosion) to the heads.
The nuts are not expensive, but the head repairs are. You will need the spanner to put them back on so it is not money wasted. New nuts will look nice.
Get a hacksaw and cut through the nut at 90 degrees to direction of rotation - do not cut all the way through to the thread - when you have a good cut, get a hammer and masonry chisel and give it a firm wack on the cut - and that should break the nut at the point of the cut and the nut will become slack and easy to remove.
The only way I would risk undoing them was if I had had assurances from a trustworthy previous owner that they have been recently been removed and were put back with some form of lube to prevent corrosion.
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