I'm pretty sure this is the way things work:
The real issue is the crankset and whether it was made before 1978 (pre CPSC) or 1978 and later. The earlier cranks take a shorter spindle, and the markings are 68-ss-120 or 70-ss-120 with the English spindle being 112mm or so and the Italian spindle being 113 or so. Post CSPC cranks need a longer spindle (~3mm and labled 68-ss or 70-ss) to accommodate the lip on the front derailleur mandated by the CPSC. So new cranks mounted on an old spindle will push on too close to the frame, old cranks on a new spindle may be slightly too far out, although as someone mentioned, with old cranks where the tapers are worn that may be ok.
None of this has anything to do with rear spacing.
All of this stuff is on the web somewhere...
> > > Campagnolo road (Strada) axle markings in Catalog 17:
> > > 68-SS-120
> > > 70-SS-120
> > FWIW, my NR Strada spindle from '85 is only marked 68-SS,
> > and the catalog No. 18 lists them that way also -- only
> > the Pista spindles are differentiated by adding 100 or
> > 120.
> By the mid-1980s, Campagnolo had lost all discipline, as far
> as bottom bracket spindle markings were concerned. The latter,
> 111mm symmetrical Athena/Chorus spindles are also marked only
> with "68-SS" or "70-SS". Also, some latter spindles, intended
> for thick wall cups, have a block oxide finish - like the
> earlier spindles meant for thin walled cups.
> But when Catalogue 17 was printed, even the demi-gods in
> Vicenza couldn't predict the confusion that would eventually
> prevail in the future.
> Fred "Spindle-ologist" Rednor - Arlington, Virginia (USA)