A dremel and a nut cracker is all you need. Cut the nuts close to the threads and then snap them off. Takes some time but easy peasy. You could drill them. You could try PB Blaster overnight soaked and I mean soaked wrapped in shop rags. Sounds like you have bimetal corrosion.
These screws are countersunk into an aluminum block, and are flush across the surface of the block. They're Allen drive, so are removed by inserting an Allen wrench into them and turning. There's nothing for me to cut off, and drilling them out would probably destoy the female threads inside the block.
I've got to get them to turn, or I'll have to buy a whole new assembly :-(