Since you are doing all the modifying, have you thought about getting disc brake tabs welded to the frame? I'm not sure how much that would cost but then you could use a set of disc brakes and any size rim you want. The only catch would be is that you have hubs that are disc brake compatible but you can pretty much lace in any hub you want to a set of rims.

Then you could play around and try all sorts of different rims and not worry about the brakes.