I would buy the 20" and have it cut to 17" or 18" myself.
Wait...I have done that before ...lol
Here is an article on the muzzle bellowing out with 5/8x24 threads.
http://bulletin.accurateshooter.com/2015/04/muzzle-threading-dont-remove-too-much-steel/
Also I ran some numbers and think it can be cut to 5/8x24 but your smith has no room for error.
When I cut, crown, and thread, I ALWAYS treat it like I have no room for error!