I've said this before, but I feel it's worth repeating because it deals with the Devil-In-the-Details relating to saddle width.
Most humans vary no more than 15% when it comes to skeletal size, and skeletal structure is what saddle width is all about. So:
The 17 is 170mm wide, edge to edge. Be aware that there is a steel horseshoe frame that runs under the saddle at the rear. No matter how hard you try, you can't break that in. So, you have to make sure that you aren't riding on it. Ok, that's simple advice, and many folks think that there's no more to it. Wrong.
Surrounding your ischial tuberosities - the sitbones - is a fleshy pad that is specifically designed to spread the pressure experienced while sitting. Think of them as being about the size of a quarter or even a fifty-cent piece. All that tissue should fit inside that steel horseshoe or you'll feel it sooner or later on your ride. I feel it at about 60+ miles on a 17. Not bad, but the saddle is just a tad too narrow. For me, a 67 feels better the longer I ride - having no real limit on the 67.
If your ichials measure about 140mm and the 17 is 170mm, you should subtract an even 25mm for the frame (about 1/2" per side) and say that your sit zone is really 145mm inside the steel frame.