I bet when you found that it was an Ah Ha moment Greg.
You need some power tools.
And if that doesn't work use a sharp chisel and a BFH. Even if you don't cut the nut clean in two you will loosen up the rust and the nut will screw off much easier.
Why not just replace the float? I have heard some folks use a brass float that is availability for mid-fifties Fords.I'll just stay with 5/16" now Phil. If it's broke, well, you know!