If I had caught this thread years ago, I'd have suggested cantis- but not those short-pull type that you'd find on cheap bikes. They're basically the missing link between cantis and linear pulls, which are prone to brake chatter(squeal). Sometimes toe-in on the pads help, sometimes them flat helps, sometimes it's the pads themselves.
All the effort going in this bike, you might as well get some horizontal dropouts brazed on, lol. You did say something about bmx cranks, they're some of the best looking cranks to me, them and old road cranks. There are two types that I know of for conversion; pro type that uses bmx sprockets(good luck finding a 44t), and replaces the bearings&cups with press in sealed bearings. The other is best described as an ashtabula 3piece, and uses ashtabula sprockets. I have both of these reserved for both of my cruisers.