It seems to be generally accepted that the K-member stamped date is very close to production date. My 280xxx Roadrunner's K-frame was stamped 155 for June 4th. Your engine cast date timing doesn't help much because there can be quite a lag from the date the block is cast and the engine actually assembled.

The K-frame number is not hard to find. It's generally front and center viewed from below. It will also have a 9 and a 1 or 2 for shift that built it.