Ive put canti post on several steel CX bikes and have used Nova's 7mm single mitre post for brazing to the front fork and thier 9mm double-mitre post for attaching to standard size/configuration rear seatstays. These resulted in around 75mm spacing and worked fine with my brakes, required very little additional prep work on the lugs for a tight mitre fit at the intended location. You could of course also get thier zero offset post and mitre them yourself (or alter the mitres as needed from either of the 7 or 9mm offset). Canti post are so cheap (way below minimim order surcharge and shipping cost) so it is probably a good idea to get an extra set to have onhand should you find that you need something slightly different.
Most all canti brake arms have some means to adjust the pads in/out to compensate for pad wear so exact width of the post placement on the frame is not critical.
Good thread on canti post spacing over here;
http://forums.mtbr.com/vintage-retro...st-399297.html