I've used the 903s on Atlas ore cars, and a 30 car train works fine. The locos and caboose use MT N couplers, with no problems, so I'll agree with Robert. If you want scale working couplers, the 903 are probably as close as you'll get.
As for McHenrys in an Intermountain box, I haven't tried it, but Accumates didn't work. The post was just a little too large, and the coupler wouldn't turn. One could ream out the holes in the couplers, but that would be tedious, not to mention weakening the coupler.