OK, I see what you were thinking. You probably know only about 10,000 bikes were made in the USA that year and half were LTD version. Those were the only bikes with S/N beginning with a 5. Far and away the majority of KZ900's were built in Japan with the old S/N designation, following the Z1B's. All of the engines were assembled in Japan and I imagine they sent them to Nebraska in lots. So yes, early S/N frames probably saw early S/N engines, but there would be likely be large gaps between engine shipments.
Originally this was called a '76 but without the S/N it's impossible to determine where it was assembled. Turns out it was a '77 so the question is moot.