Howdy Mike:
And welcome to The Forum. In addition to BIll's points, know that the front pulley system on the EOxx 200 is longer than on your '65s engine. That includes the water pump, crank pulley and other belt driven components. Check the amount of room between engine and radiator on your '65 to help you decide which way to go. Some have solved this by moving the radiator forward. Others by swapping to earlier pulleys.
Also know that the later six is rated down on compression as compared to your '65- 9:1 on the '65 200 and 8.2:1 on the EOxx. It is an easy to solve problem with machining the head. Since the newer engine had a Clifford VC it may have already had modifications. Check for paper work with the owner.
The engine upgrade is probably still a good way to go depending on the condition of your '65 engine, the condition of the EOxx, and the asking price of the upgrade engine. Check it out and keep us posted on your decision and progress.
Adios, David