The other thing to bear in mind is that races in the 1950s were of considerably longer duration, which theoretically would suggest greater field spread.
Although many of those races were, in truth, over by half-distance, and the leader(s) only needed to pace themselves to the finish and preserve their cars. So possibly those factors cancel out a bit.
There's no doubt whatsoever that pre-1970, differences in both car and driver performance were much greater than subsequently. As
Fenderman has said, budget constraints meant that winning cars were difficult to copy, and one design would often see service for several years with limited development. As for the drivers, well. Some were wealthy and of limited talent - plus ca change - but those generally weren't top 10 runners. Maybe those cars were just harder to drive...
It seems likely to me that we will see a bigger field spread in 2014, at least in the early races, as the differentials in new car performance are likely to be at their greatest, while reliability is also rumoured to be an issue. Interestingly that wasn't the case at the last major regulation change in 2009, though. Although there was diversity in designs - double-diffusers and KERS being adopted by some but not all - they translated overall into a close, competitive field.