Put it in the freezer for a bit, metal shrinks when it gets cold. That might help. When you do get it off, take all the spacers off and put only the sprocket on the shaft. Move it back and forth on the shaft till it looks lined up right, and then (without taking the sprocket off or moving it) sit different spacer combos on top of the shaft till it fills in the gap near perfect (they are different thicknesses, so you have many combos). When it's as best as it'll get take the sprocket off, put the correct spacers on first,then the sprocket, then the remaining spacers. Then screw it all back together (using the washer of course). Don't forget the set screws on the sides of the sprocket or the key.