how do i tell if my engine is

My '65 Was manufactured in December '64, with an engine cast date in October '64.

As long as the casting date within 6 months before car manufacture date, you should be good.

My parts engine, came out of a '65 Valiant, but engine casting date was Feb '66. I told the guy I got it from he'd been duped when they told him it was the original engine. It also had a cast crank, so someone had taken the steel one and replaced it.