I found a site that has a reamer for the Baby Nambu and they sent me an Excel spreadsheet with the drawing. (Not clear whether this is originally from some kind of "official" specs or drawn up from customer supplied cases.)
It is
Pacific Tool & Gauge
Phone:541-826-5808
Fax: 541-826-0773
675 Antelope Rd.
White City, OR 97503
http://pacifictoolandgauge.com/They show .273 for the bore (at least I think that is what that is measuring) so maybe my guess of .277 may be too loose.
Anyway, comparing the shoulder and neck specs with the Barnes figures for the cartridge, and the rim, base, and body specs for the .223 and Tokarev chambers with the same for the cartridge specs I should be able to come to a reasonable clearance. Or, more simply, combine the base end of .223 and Tokarev chambers with the shoulder and neck specs for the .280 Nambu chamber to determine the reamer specs I'll need.
Fun stuff!