I like the idea of using the Surly 17/19t dingle on the fixed side and White industries 20/22 freewheel on the other side. Now I'm just contemplating what chainrings might be good to use on this bike. The 54t I'm running now is super tall but the 40t (also on bike already) is probably a little on the low side. I notice most people on this forum seem to be running chainrings with number of teeth in mid 40s.

Yes, I can work with a 5t difference using my dropouts and the same chain. I'm now thinking about going with two chainrings that differ by only a couple teeth which would allow me to more comfortably utilize all the cogs and get a bit more gear reduction. I just don't have enough experience to know which gear combination will be best suited for the kind of riding I'm wanting to do (mostly flatland touring, and some commuting).