The problem with the later cars equipped with the numerically lower (i.e., 2-series) rear gears is that by that time they were all equipped with a single exhaust and an extremely restrictive pellet-bed catalytic converter. That severely limited the ability of the engine to breathe at the upper RPMs, and the HP produced at speed reflected that. The result was drag-limited top speeds at lower RPMs than you'd think. The 403s in particular were not high-RPM engines so they wouldn't make impressive numbers regardless of the rear gearing.
Few magazines tested top speed, as most accepted the manufacturer's claim rather than confirm it for themselves due to the problems and risks associated with doing so. Car & Driver magazine was the exception until recently, when legal liabilities finally forced them to suspend independent verifications on road tests. If you had the time, that'd be the source to check to see which had the best speed.
My bet would be on a 70-1/2 RAIII 4 speed with a low-3 rear gear. Those had the good exhaust manifolds, richer jetting, more aggressive timing, and true dual exhaust.
