This can be a bitch sometimes...
I usually leave the caliper in the correct spot, slide the wheel far enough forward to get the chain on, then sit facing the clutch-side of the rear wheel, with both feet supporting the front and rear of the tire (instead of the block of wood mentioned above-GREAT IDEA) then you can lift the wheel with your feet, use one hand to make sure the spacer clears and the other to push the axle thru...
its one of those things that either goes so smooth you don't even think about it or you have to call a friend to patch the hole in your garage from the wrench you threw.
Keep working with it and DO NOT SPREAD THE SWING ARM!