Hi,
Yes it is very hard (each time)
What I remember using is vice grips over the shaft to stop the shaft from retracting into the spring can. Yes that will affect the nice paint job you did (?) I would put everything together, but the tandem would not be on the spindle. Then I'll pull everything to compress the spring adding a new vice grip on the shaft as I can compress the spring. As soon as it is compressed enough, I would pull the tandem onto the spindle.
That means that the dual conical bearings are slack when I push the tandem onto the spindle, so that's a slack I use. Then, when it is on I can pull the whole thing a little bit more and then seat the bearing correctly. When screwing the washer holding the bearing together, that's how I end up putting the final force against the spring cannister.
I hope somebody has a better trick since I'd be using it as well!
jf