It's been a while since I've had my 22/45 apart but if I remember correctly you have to insert the mag, safety off, pull the trigger - then you can get the bolt to go into place. Remove the mag, insert the mainspring / stop pin, tilt the muzzle of the pistol up so that the mainspring rod goes where it should and lock it up.
It is a pain but once you get the hang of it it is pretty easy.