I am very happy with the length of my B68 and B67, I do tend to use the nose to steer a lot.
Sitting on one of Quilfred's "short" Brooks (I don't remember what it was) kind of scared me because I didn't have that control. Kinda felt like I was gonna fall, you know?
Do you feel like you have great control with your B18? The B67/B68 only have 1 cm more, so it's not a huge difference, but you might notice it.
My Surly said to tell your Surly that she loves her black B67 with black springs, and highly recommends www.wallbike.com because you have 6 months to return the saddle if the heavens don't open up and the angels sing when you ride. (my Surly is a little melodramatic.)