slow13dude.. i think you will find *why* they didn't revise things majorly over that time is that they didn't *have to*.. Why change something just for the sake of it? If there engines were doing well, and were being manufactured to cost, and being carried across a number of different models. Why spend a heap of R&D developing new engines at that time? when the money could be spent on body and chasis design, marketing, and whatever else.
I think you will find most other companies at the time have gone with similar methodology - come up with a core design that fundementally works well, and overtime revise that basic design with extra little features and extras to improve it.
If you think about it, they stacked up just as good as anything else to come from any other stable in a fairly "common" production level car over all that time. Hell, Holden, (and quite a few other companies worldwide) have only just caught up to the wows of DOHC and everything else which nissan had in their engines in the mid-80's.