Get fitted by an expert..
I'm not in the market for a C50 (wish I was). However I went through getting sized for a Master X-Light two years ago. I messed around on the internet and was put on a 58 or a 59 depending on which measurement I went with (inseams can vary depending on who takes them). I'm 6-0 34.50 inseam, short torso (I forgot my measurement, I'm too lazy to look it up).
My LBS is an authorized Colnago dealer, one of the few in NorCal.
The owner did an inshop before hours fitting at my request. He put me on a 59 (56.9 tt) with a 100mm stem and 4cm of spacers. I have long legs and my saddle is jacked up pretty high (4" bar to saddle drop). If I had gone with a 58 I would have had to jack up my saddle higher, 110 stem, but the bar to saddle drop would have been too severe for me (Campy Chorus headset, Newton stem). He built the bike with the intention of having one of the spacers removed, and the the option of swapping out stems, but I feel comfortable with the way it was built, so I did not mess with it. The only thing I have messed with a lot is my saddle position, but that probably comes with all new bikes.
It pays to get fitted by someone who is familar with the Colnago geometry, and the important things is WHO takes the measurements.
Maybe if I was young and fexible I could ride a 58, but a lot of it depends on how you ride and how you like to set your bike up. We talked about this a lot before we ordered my frame.
I knew very little about how to get fitted, but I put my faith in someone who did.
Good luck