Back in the day I used to rebuild cranks for all the local shops. I used a heavy duty and quite expensive jig to keep everything aligned while it gets pressed together. Something like this. That gets it pretty close but usually not quite true enough so the crank goes on to a truing jig like this Then is persuaded to run straight and true with a big brass mallet. I've seen folks hold them between centers to true them up but I could never make that work. Couldn't figure out where to hit it.