The bearings need to go all the way to the bottom of the steering stem - and yes, that's a bastard of a job if you don't happen to have exactly the right diameter piece of pipe knocking about to belt them on with. Or even better, a hydraulic press.
I've just done the same job on my Pan European. The interference fit between the stem and the lower race is incredibly tight - I used a variety of tools to get them down, but the most useful thing was the old bearing race lower half, which I could put over the new one to protect it when I was bearing (ha - see what I did there?) down on the whole assembly.
And don't forget, the seal with the rubber edges goes on first, then the bearing . . . been there, done that.