So I have a Bianchi Pista and I’m looking to get this fork in the threaded variety. I have a 130mm headtube and my question is do I get the 150mm steerer? Or do I have to order directly from Soma to get the 170mm steerer? I am not well versed in the ways of threaded forks, so any help is good 
get a long enough steerer tube so that you can cut it down to match your current fork.
No he wants to switch to threaded.
first don’t do this. It makes it clear to everyone who has seen a new pista and knows anything about them that you are a tool. If you must broadcast this fact about yourself you need to figure out what headset you’re going to use and find it’s stack height. You may even need longer than 170mm.
Welp, I’ve basically realized just that and I’m gonna go threadless.