I remember when you were trying to sell off the single stage press and I tried to talk you out of it, now you are talking upgrade
A Rockchucker is something you can troll for and snag when opportunity presents. BTW, my press in the photo is probably almost as old as yours, I think Dad bought it when I was about 14yo.
It is probably just gonna take some ass, after all a 62,000psi explosion took place inside the case you are squeezing.
Maybe an interim plan could be to setup the 550 for sizing. You could de-prime in the first station with a Lee decapper, they are kinda cool, they have a collet and you can set it loose so it will slide if you get a Berdan or something. What I am doing for 223 is also use a carbide neck button in a subsequent die. This is handy if you stick one, because the top of the die is open and you can drop something in it to drive it out. But then you probably have the 550 setup to do something else and don't wanna tamper with it all the time, but this could be an option.
ETA: Not sure how MUCH leverage you gain, but your shoulder (is the right side the one with issues?) might appreciate it.