ALPA may have had an input (limit clauses et al) but the FAA had a weight limit for two crew ops of (if memory serves)70,000lb. Thus BAC 1-11s for FAA customers had lower AUW/MTOW than CAA aircraft. This was gradually increased & the weights became the same on both sides of the pond.
The 767 was originally set up for 3-crew but the FAA started basing everything on workload and, with EICAS, two crew was deemed to be able to handle it. BTW Ansett & El Al 67s had 3 crew cockpits with an additional EICAS DU for the FE.