24mm is almost exactly 15/16". An impact wrench is the first choice for removing a flywheel nut, second would be to put the box end of a wrench on the nut and give the other end of the wrench a few sharp raps with a hammer. It should come loose. For tightening the nut later you do need a way to hold the flywheel. I'm not familiar with a 425 engine to give you any specific advice on that.