Fluffman,
I've just been looking through an old(ish) Colnago catalogue - and have seen this model and paint scheme ('D21B') in there - the catalogue is the 50th Anniv edition (the year the C50 was introduced), I think 2004.
Sadly, no mention of the tubing used. I've recently seen another Colnago frame (a Competition Classic), which has shaped Zona tubing, it is definitely a different model to this Classic.
Re-Stem - no, you can't use a quill stem on a threadless fork - choices are a) to find a threaded fork, b) get the current fork machined to threaded, or c) use an Ahead stem. If it were me I'd use what you have. There are some Ahead stems in silver, which would move towards a more classic look and would look really good. (I think Deda and Ritchey may do silver, others can perhaps help out with suggestions).
Looks good, I'm looking forward to a ride report,
David.