not familiar with that particular spreader- wish there was a picture. But assuming it's like a lot of other stuff from that day - a solid axle shaft with a hub that simply fits on it....
sometimes it helps pushing the wheel back further onto the axle first if it's possible.
Very often people beat on the end of the axle, trying to push it through the hub.
What that tends to do is mushroom out the end of the axle, virtually locking the hub on. (it doesn't take much)
If you push the hub back first, you can then chamfer the end of the axle and get rid of that mushroomed area. The wheel should then slide off.
Apologize if this doesn't apply to your situation - I'm just guessing.