My friend Len was the 390 engineer in 1968. All FE series engines (352, 390, 406, 427, 428...) were built at the Dearborn Engine Plant (DEP).
All engines were indeed hot tested. As Jeff says during any production year there were many variations including truck engines. Also included though were marine applications. Len told me it was very interesting when they screwed up the build and installed a marine crank (which spins the opposite direction) and fired it up.
I'll have to ask Len if they were built in batches or not. He did mention that there were computerized records of the engine as it went through the assembly and testing process.