"But why must they go to the classic first? Is it because they can't do IRO duty on the 744?"
Hard enough of a transition to go from C-152/172 to the Classic, where the former PFE already knows the systems, limitations, and most of the SOP's from reading the checklist over all those years, but still have to get the feel for actually FLYING the airplane. So why make life more difficult by adding the glass cockpit?
Plus Atlas training program on the -400 requires some jet time (2500 hours???) to get the CA type rating to be used as relief pilot, so PFE's would not qualify. A few (3?) FO's have been hired onto the -400 who do not meet this, but a lot would create scheduling problems. Most Classic flights are basic crew, so not an issue.
And do you really want the poor guy/gal to just get 1 or 2 landings a month on the -400, where they will never really get proficient? Hopefully, will get more hands-on time and cycles on the Classic.