I haven't ever done this conversion, but if you're not running brakes, the only real deciding factor is whether or not your tire choice will fit if you go wider than the original 1.25". Since you're installing a smaller rim, you should be able to size your tire up quite a fair bit, but I'd double check the clearance before buying.

Also, what is the framing spaced at, and what are the new wheels spaced at? Since it's steel, you could always cold set to make them fit, but just something to keep in mind.

I upgraded from 27" to 700c and had to spring the fork open slightly and close the rear a bit. I also has to slightly file the fork end since the original axle was slightly thinner than the replacement.