All 289 HiPo engines were assembled at the SAME location as the major pieces were cast: The Ford Cleveland Foundry and Engine Plant. It is possible that the block casting dates and final assembly dates were fairly close on regular 289 2V and 4V engines, and less likely for 289 High Performance engines.
Once the 289 engines were built and tested at Cleveland, they were crated in heavy duty containers, and shipped by rail cars to the various Ford and Mercury assembly plants as needed. Considering how many Ford and Mercury car lines used the venerable Ford Small Block, there was a HUGE production schedule at Cleveland to keep up with demand for standard 289 V-8 engines. It reached "critical mass" and maxed out (mainly due to the overwhelming demand for Mustang V-8 power), and Ford had to add additional capacity by tooling up the Windsor Foundry in 1966 to supplement the Cleveland Plant production capacity...Since the 289 High Performance represented such a small percentage of total 289 production, emphasis was not placed on HiPo production, hence the wider spread of date codes we see when examining an original 289 HiPo engine.