Dean,
I am building a Viper and I too am using a Sierra Column. However, I'm afraid that is where the similarities end. Mine is a Classic Replicas Viper (just the same as the Cobretti) but I am using the Jaguar based chassis and it looks a damn site different to yours!
I have fitted my column after taking measurements off a friends Viper, who is using the Jaguar column instead. I can't say for certain but I think that it's bloomin' close and should be alright. Have you tried pulling the column apart? The Sierras have a small amount of play that you can use to lengthen the column. I did this and I got away without having to extend it or anything.
If you e-mail me (so I get your address) I'll send you some photo's of my setup, if it helps. I'm not in your neck of the woods but if you're ever up near Donington race track you are more than welcome to call in as I live just five minutes from there.
My build page hasn't been updated since February but work has been going on. Have a look and you can e-mail me from there. The address is
http://www.fake-snake.com
Hope this helps.